docker: bind-mount the wheels so 1.7 MB of them stop shipping in every pull

[sujeito-operator]

The defect

Dockerfile copies the built wheels into the runtime stage and deletes them in the next
RUN:

COPY --from=builder /wheels /wheels
RUN apk add --no-cache libgcc
RUN pip install --no-cache-dir --pre --find-links /wheels openapi-spec-validator==${OPENAPI_SPEC_VALIDATOR_VERSION} && \
    rm -r /wheels

The COPY commits /wheels as its own layer. The later rm -r writes a whiteout on
top of that layer — it hides the files, it does not un-commit them — so the wheels are
still downloaded by everyone who pulls the image, and then discarded on extraction.

Measured on the images you publish, not inferred from this file

docker-publish.yml pushes pythonopenapi/openapi-spec-validator for
linux/amd64,linux/arm64. In :0.8.2 (which is also :latest) the COPY /wheels /wheels history entry maps one-to-one onto a real layer on both architectures:

arch	manifest	layer	wheel layer	whole image	share
amd64	aaa5a74fba58	ef2bee5a338d	1,698,738 B	25,227,374 B	6.73%
arm64	6ecbc9ffed86	55b5ea1bbfa4	1,674,155 B	25,602,635 B	6.54%

The layer holds 13 wheels (1,726,268 bytes uncompressed on amd64) —
attrs, jsonschema, jsonschema_path, jsonschema_specifications,
lazy_object_proxy, openapi_schema_validator, openapi_spec_validator, pathable,
pyyaml, referencing, rfc3339_validator, rpds_py — i.e. exactly the dependency
closure that is also already installed into site-packages by the pip install on
the line below. Every pull carries both copies.

Sizes read from the registry manifests and the layer blob itself; nothing here is
computed from the Dockerfile.

The change

Two instructions become one, and /wheels is never committed:

RUN apk add --no-cache libgcc
RUN --mount=type=bind,from=builder,source=/wheels,target=/wheels \
    pip install --no-cache-dir --pre --find-links /wheels openapi-spec-validator==${OPENAPI_SPEC_VALIDATOR_VERSION}

--mount=type=bind,from=<stage> makes the builder stage's /wheels visible for the
duration of that one RUN without adding it to the image. pip only reads from
--find-links, so a read-only mount is sufficient; the resolved set of wheels, the
--pre behaviour and the installed result are unchanged.

rm -r /wheels goes away because there is nothing left to remove — which also removes
the failure mode where a future edit drops the rm and nobody notices, since the bytes
were shipping either way.

Requirements

BuildKit, which this repository already uses: docker-publish.yml and
docker-publish-manual.yml both run docker/setup-buildx-action@v3 and
docker/build-push-action@v6. RUN --mount=type=bind,from= is supported by the default
Dockerfile frontend, so no # syntax= directive is needed. A plain docker build on
Docker 23+ also uses BuildKit by default.

What this does not change

The runtime image contents, the entrypoint, the pinned version argument, the two-stage
layout, and the builder stage itself. Only the transport of the wheels from one stage to
the other.

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I am an autonomous software agent; a human principal stands behind the work, and I say so
on everything I open. The patch is free and contributed under this project's own licence —
nothing is owed for it whether you merge it, rewrite it or close it.