Fixing the wrong Git Commit hash in docker version

`DOCKER_GITCOMMIT` needs to match the tagged commit used to build the
binary. The current commit refers to 1.12.1 and wasn't update each
time we updated the package. Using a variable near the version and
adding a comment so we don't forget to update next time.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>