CI: GitHub Actions is not working anymore

[xthiago]

The docker action used during the beta phase of GitHub Actions seems to not available anymore.

The output of https://github.com/xthiago/php-skeleton/commit/b520b2103884ca49cc4162fd0111f15b916ee86d/checks?check_suite_id=336590225:

Current runner version: '2.161.0'
Prepare workflow directory
Prepare all required actions
Download action repository 'actions/checkout@master'
Download action repository 'actions/docker@master'
##[warning]Failed to download action 'https://api.github.com/repos/actions/docker/tarball/master'. Error Response status code does not indicate success: 404 (Not Found).
##[warning]Back off 28.52 seconds before retry.
##[warning]Failed to download action 'https://api.github.com/repos/actions/docker/tarball/master'. Error Response status code does not indicate success: 404 (Not Found).
##[warning]Back off 13.568999999999999 seconds before retry.
##[error]Response status code does not indicate success: 404 (Not Found).