Add STOPSIGNAL to ensure clean shutdown of container

[manics]

Without this docker stop will timeout (default 10s) and automatically kill the container. With this docker stop should cleanly shutdown the container.

[joshmoore]

Docker for Mac (1.12.3-beta30):

Without this PR:

# joshuas-mbp:omero-ssh-c7-docker jamoore$ docker run --name tt -d --privileged test-ssh-c7
308a3b969a9d8c55178ed18f5ec1000d949db998bb890a3bbecaa79313db394b
# joshuas-mbp:omero-ssh-c7-docker jamoore$ docker
# joshuas-mbp:omero-ssh-c7-docker jamoore$ time docker stop tt
tt

real    0m10.424s
user    0m0.008s
sys 0m0.008s

# Rebuild...

Step 12 : STOPSIGNAL "RTMIN+3"
 ---> Running in 4762dab0f2f6
 ---> 3b1d4723590d
Removing intermediate container 4762dab0f2f6
Successfully built 3b1d4723590d
joshuas-mbp:omero-ssh-c7-docker jamoore$ docker run --name tt -d test-ssh-c7
c6c913070645c735ade61ee7ccec2151beac15dd0ac10b32bcf87a7a0af25e66
# joshuas-mbp:omero-ssh-c7-docker jamooretime docker stop tt
tt

real    0m0.570s
user    0m0.007s
sys 0m0.007s

NB: Only works if --privileged is passed of course.

👍