Give each platform its own uv cache in the image build

The parallel per-platform builds shared one uv cache mount, so they
queued on uv's cache lock. When uv.lock changed, the emulated armv7
sync held it long enough for the others to hit uv's 300 s lock timeout
and fail publish-images.

Dockerfile

@@ -1,5 +1,12 @@
 FROM python:3.14-slim-bookworm AS python-builder
 
+# A multi-platform build runs this stage once per platform in parallel. Each gets
+# its own uv cache: wheels are per-platform anyway, and one shared cache made the
+# platforms queue on uv's cache lock until the slow emulated armv7 sync pushed the
+# others past uv's 300 s lock timeout whenever uv.lock changed.
+ARG TARGETARCH
+ARG TARGETVARIANT
+
 ENV UV_COMPILE_BYTECODE=1
 ENV UV_LINK_MODE=copy
 ENV UV_NO_DEV=1
@@ -21,7 +28,7 @@ RUN pip install --no-cache-dir uv
 
 # Install dependencies
 WORKDIR /home/app/spoolman
-RUN --mount=type=cache,target=/root/.cache/uv \
+RUN --mount=type=cache,id=uv-${TARGETARCH}${TARGETVARIANT},target=/root/.cache/uv \
     --mount=type=bind,source=uv.lock,target=uv.lock \
     --mount=type=bind,source=pyproject.toml,target=pyproject.toml \
     uv sync --locked --no-install-project \
@@ -45,7 +52,7 @@ RUN if [ "$(uname -m)" = "armv7l" ]; then \
 COPY migrations /home/app/spoolman/migrations
 COPY spoolman /home/app/spoolman/spoolman
 COPY alembic.ini README.md uv.lock pyproject.toml /home/app/spoolman/
-RUN --mount=type=cache,target=/root/.cache/uv \
+RUN --mount=type=cache,id=uv-${TARGETARCH}${TARGETVARIANT},target=/root/.cache/uv \
     uv sync --locked \
     && uv cache prune --ci