docker: ask s6 to stop all service when gitea stop

[sapk]

This should fix #9164 part of the container not stopping when gitea fail.

Follow : https://github.com/just-containers/s6-overlay#writing-an-optional-finish-script

Still WIP since I need to test it.

[sapk]

Tested (and fixed) !

Before :

$ docker run -v "$(pwd)/data:/data:ro" --rm -ti gitea/gitea
....
2019/11/27 01:19:20 ...s/setting/setting.go:670:NewContext() [F] Failed to create '/data/git/.ssh': mkdir /data/git: read-only file system
mkdir: can't create directory '/data/ssh': Read-only file system
...
2019/11/27 01:19:22 ...s/setting/setting.go:670:NewContext() [F] Failed to create '/data/git/.ssh': mkdir /data/git: read-only file system
mkdir: can't create directory '/data/ssh': Read-only file system
...
2019/11/27 01:19:22 ...s/setting/setting.go:670:NewContext() [F] Failed to create '/data/git/.ssh': mkdir /data/git: read-only file system
...

After :

$ docker run -v "$(pwd)/data:/data:ro" --rm -ti sapk/gitea
...
2019/11/27 01:46:27 cmd/web.go:102:runWeb() [I] Starting Gitea on PID: 16
2019/11/27 01:46:27 ...s/setting/setting.go:523:NewContext() [W] Custom config '/data/gitea/conf/app.ini' not found, ignore this if you're running first time
2019/11/27 01:46:27 ...s/setting/setting.go:670:NewContext() [F] Failed to create '/data/git/.ssh': mkdir /data/git: read-only file system
Saving key "/data/ssh/ssh_host_rsa_key" failed: No such file or directory
Generating /data/ssh/ssh_host_dsa_key...

All services are stopped quickly after first fail and the container is stopped.

[codecov-io]

Codecov Report

Merging #9171 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9171      +/-   ##
==========================================
+ Coverage   41.03%   41.03%   +<.01%     
==========================================
  Files         553      553              
  Lines       72041    72041              
==========================================
+ Hits        29564    29565       +1     
- Misses      38744    38746       +2     
+ Partials     3733     3730       -3

Impacted Files	Coverage Δ
modules/indexer/indexer.go	44.73% <0%> (-10.53%)	⬇️
modules/log/event.go	65.64% <0%> (+1.02%)	⬆️
modules/process/manager.go	78.37% <0%> (+4.05%)	⬆️

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