add parents option to dockerfile COPY command

[wclr]

Docs on COPY command state:

If <src> is any other kind of file, it is copied individually along with its metadata. In this case, 
if <dest> ends with a trailing slash /, 
it will be considered a directory and the contents of <src> will be written at <dest>/base(<src>).

I think it would be useful to have be able to have as a copy result <dest>/<src>.

This would allow combining in one COPY command files from different folders in the context, this can be useful when trying to optimize for example node apps builds:

# would place files to /app/package1/package.json & /app/some/dep
COPY some/dep package1/package.json /app/ --parents 
RUN cd package1 && npm install

# would place files to /app/package2/package.json & /app/other/dep
COPY other/dep/ package2/package.json /app/ --parents 
RUN cd package2 && npm install

Currently, have to split COPY commands for this case or do something else less optimal.

[thaJeztah]

Related discussion in BuildKit: moby/buildkit#396

[nwalters512]

This is very much needed with the rise of Node monorepos. If I have a packages/ directory with many subdirectories, each containing a package.json, I need all those package.json files to be present when I run yarn install/npm install. Right now, I have two options:

• Use individual COPY commands for each package, which is inexplicably slow on GitHub Actions and also a maintenance burden.
• COPY the entire packages/ directory, which eliminates most of the benefits of Docker layer caching as the layer cache will be invalidated for most builds.

If COPY --parents existed, I believe one could do something like this and take full advantage of layer caching and the speed of a single COPY command:

COPY --parents packages/*/package.json apps/*/package.json /path/to/app/
# The container should now have `/path/to/app/packages/foo/package.json`,
# `/path/to/app/packages/bar/package.json`,
# `/path/to/app/apps/baz/package.json`, and so on.

[thaJeztah]

There's a work-in-progress pull request in BuildKit implementing this;

• Add --parents opt-in flag for ADD/COPY (labs) buildkit#3001