Added docs for using GitHubActions caching to speedup CI

[staabm]

closes https://github.com/FriendsOfPHP/PHP-CS-Fixer/issues/4938

[staabm]

ohh my.. getting a green build for a README.rst change is a real pain in the ass.

[SpacePossum]

Thanks for working on this! I haven't used github action yet so I cannot comment on the content itself. The readme is already very long IMHO, therefore I'm +0 on this change. I would love to see it be added to https://github.com/FriendsOfPHP/PHP-CS-Fixer/pull/3475 as that provides more dedicated doc files by topic.

[kubawerlos]

ohh my.. getting a green build for a README.rst change is a real pain in the ass.

@staabm what is the problem? It is very easy.

[jackbentley]

For me, this cache solution has 3 problems:

• It's very inefficient and potentially useless when it comes to concurrent multi-branch development (e.g. multiple devs working on lots of branches)
• Time/storage/etc is still spent on creating cache when no files are changed that would affect the run of PHP CS Fixer
• It requires a separate directory for the cache file. GitHub Actions cache v2 is capable of path representing a file - so this example seems weirdly overcomplicated for no reason.

Something that has a lock file such as composer doesn't have these issues because they don't get updated too frequently and hits the primary key so a new cache file isn't created every commit.

Implementing cache for PHP CS Fixer previously, I took a branch and file hash based approach. For example:

      - uses: actions/cache@v2
        with:
          path: .php_cs.cache
          key: ${{ runner.os }}-php-cs-fixer-${{ github.ref }}-${{ hashFiles('**/src/**', '**/tests/**') }}
          restore-keys: |
            ${{ runner.os }}-php-cs-fixer-${{ github.ref }}-
            ${{ runner.os }}-php-cs-fixer-refs/heads/main-

This will use the latest cache for that branch, then will failover to the main branch's latest cache. It will also only create a new cache when there are changes to files inside one of the directories which PHP CS Fixer will analyse - in this case src and tests.

There are probably other improvements that could be made or considered also, such as including the hash of the config file.