Increase Docker stop timeout to 2m and improve error messaging

See merge request !6868



Merged-by: Axel von Bertoldi <avonbertoldi@gitlab.com>
Approved-by: Georgi N. Georgiev | GitLab <ggeorgiev@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: Daniel Keenan <dkeenan@gitlab.com>

(cherry picked from commit 1e0edf1d)

250e494e Add ErrStopKillWaitTimeout sentinel
312b796a Bump stop-kill wait timeout to 2m
7120e04d Trim comments
dd1602d4 Update comment and error message
766e3dc8 Return ErrStopKillWaitTimeout when appropriate
3096bf38 Include elapsed time in returned error
154b0b17 Remove stale comment
3c32ce1f Fix typo in gracefulExitFunc
2a96b5aa Move stopKillTimeout to dockerWaiter
c67d0cf0 Add unit test TestDockerWaiter_StopKillWait_TimeoutWraps
7284e007 Tweak comment
f07fb106 Refactor
e4132666 Refactor to use context.WithTimeoutCause

Co-authored-by: Axel von Bertoldi <avonbertoldi@gitlab.com>