buildctl: Use CSV style for `--frontend` and `--export`

[AkihiroSuda]

#615 (comment)

Frontend

• Old style: --frontend dockerfile.v0 --frontend-opt target=foo --frontend-opt build-arg:foo=bar
• New style: --frontend type=dockerfile.v0,target=foo,build-arg:foo=bar

Export

• Old style: --exporter image --exporter-opt name=docker.io/username/image --exporter-opt push=true
• New style: --export type=image,name=docker.io/username/image,push=true
  • s/exporter/export/ for consistency with --cache-export, probably
  • Multiple --export should be supported

[tonistiigi]

SGTM

@thaJeztah @tiborvass @ijc

[thaJeztah]

Thanks! New style generally looks good; some questions / thoughts:

• Do we want to keep the old style supported as well (i.e., keep --exporter=image as a shorthand for --exporter type=image)?
• Not new, but build-arg:foo=bar caught my eye, because (in docker cli) we deprecated the : syntax for --security-opt foo:bar and some other options. Using = may be a bit more challenging for the CSV notation though (--frontend type=dockerfile.v0,build-arg=FOOBAR=baz); also taking into account that --build-arg FOOBAR (without a value) should take the value from $FOOBAR if set as environment variable.

[ijc]

Fine by me. I assume there is a way to escape a comma if you need one in a value?

[thaJeztah]

I think it's like this;

--foobar option=foo,"otheroption=one,two,three"

If there's gonna be many options to set, we should perhaps start thinking about a build-file (as could be done with the docker-compose v2 file format, but targeted at build, instead of trying to mix "build" and "deploy" in the same file format).

[AkihiroSuda]

Yes, I believe build-file is a must-to-have feature either way 👍

[tonistiigi]

@AkihiroSuda This is the last open issue in v0.4.0 milestone

[AkihiroSuda]

I'll work on this after FOSDEM