Improve Docker build reliability and add build idempotency:

Dockerfile

@@ -74,12 +74,18 @@ RUN ln -sf ~/.local/bin/mkosi /usr/bin/mkosi
 # Verify mkosi is functional
 RUN mkosi --version
 
-# Prime uv's cache with our pyproject.toml to speed up runtime
-COPY pyproject.toml /tmp/pyproject.toml
-COPY captain /tmp/captain
-COPY build.py /tmp/build.py
-WORKDIR /tmp
-RUN uv --verbose run build.py --help
+# Install project dependencies into a persistent venv so that
+# `uv run` inside the container reuses it instead of recreating one.
+COPY pyproject.toml /opt/captain/pyproject.toml
+COPY uv.lock /opt/captain/uv.lock
+COPY captain /opt/captain/captain
+COPY build.py /opt/captain/build.py
+RUN uv venv /opt/captain-venv && \
+    UV_PROJECT_ENVIRONMENT=/opt/captain-venv uv sync --frozen --project /opt/captain
+
+# Point uv at the pre-built venv for all future runs.
+ENV VIRTUAL_ENV=/opt/captain-venv
+ENV UV_PROJECT_ENVIRONMENT=/opt/captain-venv
 
 WORKDIR /work
 ENTRYPOINT ["mkosi"]

captain/cli/_stages.py

@@ -102,6 +102,13 @@ def _build_mkosi_stage(cfg: Config, extra_args: list[str]) -> None:
         log.info("MKOSI_MODE=skip — skipping image assembly")
         return
 
+    # --- idempotency --------------------------------------------------
+    existing = sorted(cfg.initramfs_output.glob("*.cpio*"))
+    force = "--force" in cfg.mkosi_args
+    if existing and not force:
+        log.info("Initramfs already built: %s (use --force to rebuild)", existing[0])
+        return
+
     mkosi_args = list(cfg.mkosi_args) + list(extra_args)
 
     # --- native -------------------------------------------------------

captain/docker.py

@@ -59,7 +59,7 @@ def build_builder(cfg: Config) -> None:
         return
 
     log.info("Building Docker image '%s'...", cfg.builder_image)
-    cmd = ["docker", "buildx", "build"]
+    cmd = ["docker", "buildx", "build", "--load"]
     if cfg.no_cache:
         cmd.append("--no-cache")
     cmd.extend(
@@ -88,7 +88,7 @@ def build_release_image(cfg: Config) -> None:
         return
 
     log.info("Building Docker image '%s'...", RELEASE_IMAGE)
-    cmd = ["docker", "buildx", "build", "-f", str(cfg.project_dir / "Dockerfile.release")]
+    cmd = ["docker", "buildx", "build", "--load", "-f", str(cfg.project_dir / "Dockerfile.release")]
     if cfg.no_cache:
         cmd.append("--no-cache")
     cmd.extend(["--progress=plain"])