dockerfile: experimental ADD/COPY --chmod=...

[sj26]

I know there's a rich history for adding an ADD/COPY --chmod=... flag similar to the --chown=... flag to the Dockerfile syntax, but I don't see a feature request here in this repo where it seems to belong — could we amend the Dockerfile experimental frontend syntax to add a --chmod=0755 style flag to ADD and COPY commands which populates the mode in the associated FileOp (from #809)? I'd be happy to prototype an implementation if there were appetite.

The main use case is building thin images by adding binaries directly from external sources which don't have a correct mode, like:

FROM scratch
ADD --chmod=0755 https://github.com/pganalyze/collector/releases/download/v0.21.0/pganalyze-collector-linux-amd64 /pganalyze-collector
COMMAND /pganalyze-collector

Currently, the only ways to really do this is download the file outside the Dockerfile, set modes, then copy it in, or to use an intermediate container with a whole lot of extra weight just to use a chmod command and then copy to a subsequent FROM scratch stage.

[tonistiigi]

could we amend the Dockerfile experimental frontend syntax to add a --chmod=0755 style flag to ADD and COPY commands which populates the mode in the associated FileOp (from #809)?

Yes, this is possible. No LLB/solver changes should be required.

[tonistiigi]

#1492