Allow disabling cache via command line

[wyqydsyq]

Re-open of #1896

There is a very valid use case for wanting to completely disable the cache - Docker.

Best practices for building slim Docker images are to disable or clear caches for all package managers. There is absolutely zero benefit from having cached packages in a Docker image, it just consumes space.

We can of course delete the cache files after in a seperate command - however it seems pretty standard for modern package managers to now offer this as a CLI option, see: PIP, APK, Yarn is also looking at implementing it.

[alcohol]

I do not understand how Docker is "a very valid" use-case. I use Docker extensively and the Composer cache is not an issue, especially since multi-stage builds were introduced. Your production container should only contain your build artifact, there shouldn't be anything related to Composer inside of it if you are doing it right.

[Dzhuneyt]

Already solved in pull request #7880

Feel free to review the code and mark it as approved if you think it makes sense. That would bring it closer to release.

[alcohol]

The above can already be achieved by running, just FYI

COMPOSER_CACHE_DIR=/dev/null composer <command>

[Dzhuneyt]

I agree, but following the discussion in #6650 and more specifically - your comment (#6650 (comment)), I was under the impression that such a feature will be useful.

@Fleshgrinder correctly pointed out in his comment

Users need to find that out on their own, instead of simply getting the answer from composer --help.

I believe that having this feature as "built-in" AND crossplatform out of the box, is much better than resorting to undocumented tricks with setting environment variables in the command line.

[stof]

environment variables supported by Composer are documented: https://getcomposer.org/doc/03-cli.md#environment-variables

[alcohol]

Indeed, they are documented and they are also cross platform.. anyway, a flag that does exactly the same does not hurt I guess. It's only a few lines of code. And if it makes some people happy for some reason, that's a win I guess 🤷‍♂️

[Seldaek]

Fixed by #7880