Various usability fixes

This fixes a number of random bugs observed when using GCK across time:

• Add support many alt_stores for Redis => retains backward compatibility
• If add git group (with given GID) fails still add with a random GID
• Fix gitlab-shell log file location => otherwise cannot git push
• Update docker:dind to latest version => since the 19.03 no longer works
• Configure gitaly and gitlab-shell to use workhorse => since they sometimes require Workhorse to function
• Execute spring.sh instead of cat to provide graceful kill => since this helps in some situations and provides a way to better control the process lifecycle (especially in context of !228 (diffs))