Support XDG user directories
Description
Gitlab runner currently uses ~/.gitlab-runner directory for configuration file. On Unix systems XDG Base Directories standard has been introduced to declutter home directory, and decouple configuration from other kinds of data, and allow users to configure where files are stored in easy manner.
Proposal
Gitlab runner should respect XDG_CONFIG_HOME
environent variable (defaulting to ~/.config
if it is not defined), and store its configuration there. After this change, configuration would be stored in ~/.config/gitlab-runner/
on default. In order to maintain backward compatibility, it is advised to check if ~/.gitlab-runner
exists, and use it in such case. Otherwise, configuration following XDG standard should be honoured.