diff --git a/core/container/src/runtime.rs b/core/container/src/runtime.rs
index 7f748899..61e7f801 100644
--- a/core/container/src/runtime.rs
+++ b/core/container/src/runtime.rs
@@ -1,4 +1,4 @@
-use crate::manifest::AppManifest;
+use crate::manifest::{AppManifest, BuildConfig};
 use crate::podman_client::{ContainerState, ContainerStatus, PodmanClient};
 use anyhow::{Context, Result};
 use async_trait::async_trait;
@@ -20,6 +20,22 @@ pub trait ContainerRuntime: Send + Sync {
     async fn get_container_status(&self, name: &str) -> Result<ContainerStatus>;
     async fn get_container_logs(&self, name: &str, lines: u32) -> Result<Vec<String>>;
     async fn list_containers(&self) -> Result<Vec<ContainerStatus>>;
+
+    /// Check whether an image reference exists in local storage.
+    ///
+    /// The reconciler calls this before deciding to build. `true` means
+    /// `image inspect <image_ref>` succeeded (or equivalent); `false` means
+    /// the image is not present. Registry/network state is explicitly NOT
+    /// consulted — this is a local-storage check only.
+    async fn image_exists(&self, image_ref: &str) -> Result<bool>;
+
+    /// Build a local image from a `BuildConfig`.
+    ///
+    /// Equivalent to `podman build -t <tag> -f <dockerfile> [--build-arg K=V ...] <context>`.
+    /// The resulting image is referenceable by `config.tag` for subsequent
+    /// `create_container` / `image_exists` calls. Stdout/stderr are collected
+    /// and included in the error on failure; on success they are discarded.
+    async fn build_image(&self, config: &BuildConfig) -> Result<()>;
 }
 
 pub struct PodmanRuntime {
@@ -32,6 +48,17 @@ impl PodmanRuntime {
             client: PodmanClient::new(user),
         }
     }
+
+    /// Run `podman <args>`, returning an error with captured stderr on non-zero
+    /// exit. Used for operations (build, image inspect) that are awkward over the
+    /// HTTP API. The daemon runs as the target user already, so no sudo hop.
+    async fn podman_cli(&self, args: &[&str]) -> Result<std::process::Output> {
+        let mut cmd = TokioCommand::new("podman");
+        cmd.args(args);
+        cmd.output()
+            .await
+            .with_context(|| format!("failed to execute podman {}", args.join(" ")))
+    }
 }
 
 #[async_trait]
@@ -79,6 +106,64 @@ impl ContainerRuntime for PodmanRuntime {
     async fn list_containers(&self) -> Result<Vec<ContainerStatus>> {
         self.client.list_containers().await
     }
+
+    async fn image_exists(&self, image_ref: &str) -> Result<bool> {
+        // `podman image exists` returns 0 if present, 1 if absent. Any other
+        // exit code is an environment failure we should surface.
+        let output = self.podman_cli(&["image", "exists", image_ref]).await?;
+        match output.status.code() {
+            Some(0) => Ok(true),
+            Some(1) => Ok(false),
+            Some(code) => {
+                let stderr = String::from_utf8_lossy(&output.stderr);
+                Err(anyhow::anyhow!(
+                    "podman image exists {image_ref} exited with {code}: {stderr}"
+                ))
+            }
+            None => Err(anyhow::anyhow!(
+                "podman image exists {image_ref} terminated by signal"
+            )),
+        }
+    }
+
+    async fn build_image(&self, config: &BuildConfig) -> Result<()> {
+        let args = build_args_for_podman(config);
+        let borrowed: Vec<&str> = args.iter().map(|s| s.as_str()).collect();
+        let output = self.podman_cli(&borrowed).await?;
+        if !output.status.success() {
+            let stderr = String::from_utf8_lossy(&output.stderr);
+            let stdout = String::from_utf8_lossy(&output.stdout);
+            return Err(anyhow::anyhow!(
+                "podman build -t {} failed: {stderr}{}{stdout}",
+                config.tag,
+                if stderr.is_empty() || stdout.is_empty() { "" } else { "\n---stdout---\n" }
+            ));
+        }
+        Ok(())
+    }
+}
+
+/// Build the argv for `podman build` from a BuildConfig.
+///
+/// Extracted so it can be unit-tested without actually invoking podman.
+/// Order is fixed for deterministic tests: subcommand, -t, -f, build-args
+/// (sorted by key), context.
+fn build_args_for_podman(config: &BuildConfig) -> Vec<String> {
+    let mut args: Vec<String> = vec![
+        "build".to_string(),
+        "-t".to_string(),
+        config.tag.clone(),
+        "-f".to_string(),
+        config.dockerfile.clone(),
+    ];
+    let mut kv: Vec<(&String, &String)> = config.build_args.iter().collect();
+    kv.sort_by(|a, b| a.0.cmp(b.0));
+    for (k, v) in kv {
+        args.push("--build-arg".to_string());
+        args.push(format!("{k}={v}"));
+    }
+    args.push(config.context.clone());
+    args
 }
 
 pub struct DockerRuntime {
@@ -188,7 +273,13 @@ impl ContainerRuntime for DockerRuntime {
             cmd.arg("--cap-add").arg(cap);
         }
 
-        cmd.arg(&manifest.app.container.image);
+        let image_ref = manifest.app.container.image_ref().ok_or_else(|| {
+            anyhow::anyhow!(
+                "container config for {} has neither a valid image nor build source",
+                manifest.app.id
+            )
+        })?;
+        cmd.arg(&image_ref);
 
         let output = cmd.output().await.context("Failed to create container")?;
 
@@ -344,6 +435,42 @@ impl ContainerRuntime for DockerRuntime {
 
         Ok(result)
     }
+
+    async fn image_exists(&self, image_ref: &str) -> Result<bool> {
+        // `docker image inspect` exits 1 when the image is absent. Any message
+        // to stderr in that case is informational; we swallow it.
+        let mut cmd = self.docker_async();
+        cmd.arg("image").arg("inspect").arg(image_ref);
+        let output = cmd.output().await.context("failed to execute docker image inspect")?;
+        match output.status.code() {
+            Some(0) => Ok(true),
+            Some(1) => Ok(false),
+            Some(code) => {
+                let stderr = String::from_utf8_lossy(&output.stderr);
+                Err(anyhow::anyhow!(
+                    "docker image inspect {image_ref} exited with {code}: {stderr}"
+                ))
+            }
+            None => Err(anyhow::anyhow!(
+                "docker image inspect {image_ref} terminated by signal"
+            )),
+        }
+    }
+
+    async fn build_image(&self, config: &BuildConfig) -> Result<()> {
+        let mut cmd = self.docker_async();
+        cmd.arg("build").arg("-t").arg(&config.tag).arg("-f").arg(&config.dockerfile);
+        for (k, v) in &config.build_args {
+            cmd.arg("--build-arg").arg(format!("{k}={v}"));
+        }
+        cmd.arg(&config.context);
+        let output = cmd.output().await.context("failed to execute docker build")?;
+        if !output.status.success() {
+            let stderr = String::from_utf8_lossy(&output.stderr);
+            return Err(anyhow::anyhow!("docker build -t {} failed: {stderr}", config.tag));
+        }
+        Ok(())
+    }
 }
 
 pub struct AutoRuntime {
@@ -415,7 +542,91 @@ impl ContainerRuntime for AutoRuntime {
     async fn list_containers(&self) -> Result<Vec<ContainerStatus>> {
         self.runtime.list_containers().await
     }
+
+    async fn image_exists(&self, image_ref: &str) -> Result<bool> {
+        self.runtime.image_exists(image_ref).await
+    }
+
+    async fn build_image(&self, config: &BuildConfig) -> Result<()> {
+        self.runtime.build_image(config).await
+    }
 }
 
 // Runtime factory functions will be provided by the archipelago crate
 // that imports this library and has access to Config
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::collections::HashMap;
+
+    fn cfg(context: &str, tag: &str, dockerfile: &str, args: &[(&str, &str)]) -> BuildConfig {
+        BuildConfig {
+            context: context.to_string(),
+            dockerfile: dockerfile.to_string(),
+            tag: tag.to_string(),
+            build_args: args
+                .iter()
+                .map(|(k, v)| (k.to_string(), v.to_string()))
+                .collect::<HashMap<_, _>>(),
+        }
+    }
+
+    #[test]
+    fn build_args_minimal() {
+        let c = cfg("/tmp/ctx", "archy-bitcoin-ui:local", "Dockerfile", &[]);
+        assert_eq!(
+            build_args_for_podman(&c),
+            vec![
+                "build",
+                "-t",
+                "archy-bitcoin-ui:local",
+                "-f",
+                "Dockerfile",
+                "/tmp/ctx",
+            ]
+        );
+    }
+
+    #[test]
+    fn build_args_custom_dockerfile() {
+        let c = cfg("/opt/archy/bitcoin-ui", "x:local", "Dockerfile.prod", &[]);
+        let got = build_args_for_podman(&c);
+        assert_eq!(got[3], "-f");
+        assert_eq!(got[4], "Dockerfile.prod");
+        assert_eq!(got.last().unwrap(), "/opt/archy/bitcoin-ui");
+    }
+
+    #[test]
+    fn build_args_are_sorted_deterministically() {
+        // HashMap iteration order is nondeterministic; the runtime sorts so that
+        // equivalent BuildConfigs produce identical commands (easier to debug,
+        // cache-friendly if we ever layer build-cache keys on top).
+        let c = cfg(
+            "/c",
+            "t",
+            "Dockerfile",
+            &[("BAR", "2"), ("FOO", "1"), ("BAZ", "3")],
+        );
+        let args = build_args_for_podman(&c);
+        let flat: Vec<&str> = args.iter().map(|s| s.as_str()).collect();
+        // Build args appear as pairs of --build-arg K=V; locate them:
+        let mut pairs: Vec<&str> = Vec::new();
+        for w in flat.windows(2) {
+            if w[0] == "--build-arg" {
+                pairs.push(w[1]);
+            }
+        }
+        assert_eq!(pairs, vec!["BAR=2", "BAZ=3", "FOO=1"]);
+    }
+
+    #[test]
+    fn build_args_context_is_last() {
+        // Context MUST be the final positional argument — podman treats any
+        // stray trailing arg after build-args as the context, so placement
+        // matters. Regression guard.
+        let c = cfg("/final/context", "t", "Dockerfile", &[("K", "V")]);
+        let args = build_args_for_podman(&c);
+        assert_eq!(args.last().unwrap(), "/final/context");
+    }
+}