hack: aggregate protobuf includes to a separate stage

This moves the aggregation of the various protobuf includes (binaries
and include files) to a separate stage so the files can be retrieved
more easily by external tools.

Since the canonical way of retrieving the protobuf dependencies is to
use this file and the dependencies can be quite complicated to retrieve,
this makes it easier for outside contributors to retrieve the
dependencies and copy them into their own repositories.

This was possible before by using the `tools` target, but that would
also include the entire `golang` image. Now you can retrieve just the
`protobuf` target to get the binaries and the include files.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

Author: jsternberg

hack/dockerfiles/generated-files.Dockerfile

@@ -43,7 +43,8 @@ RUN --mount=type=bind,source=go.mod,target=/app/go.mod \
   set -e
   mkdir -p /opt/vtprotobuf
   go mod download github.com/planetscale/vtprotobuf
-  cp -r $(go list -m -f='{{.Dir}}' github.com/planetscale/vtprotobuf)/include /opt/vtprotobuf
+  cp -R $(go list -m -f='{{.Dir}}' github.com/planetscale/vtprotobuf)/include /opt/vtprotobuf
+  chmod -R 0755 /opt/vtprotobuf
 EOT
 
 FROM gobuild-base AS vendored
@@ -53,6 +54,12 @@ RUN --mount=type=bind,source=vendor,target=/app <<EOT
   find . -name '*.proto' | tar -cf - --files-from - | tar -C /opt/vendored/include -xf -
 EOT
 
+FROM scratch AS protobuf
+COPY --link --from=protoc /opt/protoc /
+COPY --link --from=googleapis /opt/googleapis /
+COPY --link --from=vtprotobuf /opt/vtprotobuf /
+COPY --link --from=vendored /opt/vendored /
+
 FROM gobuild-base AS tools
 RUN --mount=type=bind,source=go.mod,target=/app/go.mod \
     --mount=type=bind,source=go.sum,target=/app/go.sum \
@@ -62,10 +69,7 @@ RUN --mount=type=bind,source=go.mod,target=/app/go.mod \
     github.com/planetscale/vtprotobuf/cmd/protoc-gen-go-vtproto \
     google.golang.org/grpc/cmd/protoc-gen-go-grpc \
     google.golang.org/protobuf/cmd/protoc-gen-go
-COPY --link --from=protoc /opt/protoc /usr/local
-COPY --link --from=googleapis /opt/googleapis /usr/local
-COPY --link --from=vtprotobuf /opt/vtprotobuf /usr/local
-COPY --link --from=vendored /opt/vendored /usr/local
+COPY --link --from=protobuf / /usr/local
 
 FROM tools AS generated
 RUN --mount=type=bind,target=github.com/moby/buildkit <<EOT