diff --git a/Dockerfile b/Dockerfile
index 07061d5..050564d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -81,10 +81,11 @@ RUN mkosi --version
 # 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 && \
-    VIRTUAL_ENV=/opt/captain-venv uv pip install --project /opt/captain /opt/captain
+    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
diff --git a/captain/cli/_stages.py b/captain/cli/_stages.py
index d15284e..0f5c3ac 100644
--- a/captain/cli/_stages.py
+++ b/captain/cli/_stages.py
@@ -103,10 +103,10 @@ def _build_mkosi_stage(cfg: Config, extra_args: list[str]) -> None:
         return
 
     # --- idempotency --------------------------------------------------
-    initramfs_image = cfg.initramfs_output / "image.cpio.zst"
+    existing = sorted(cfg.initramfs_output.glob("*.cpio*"))
     force = "--force" in cfg.mkosi_args
-    if initramfs_image.is_file() and not force:
-        log.info("Initramfs already built: %s (use --force to rebuild)", initramfs_image)
+    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)