Remove apt-get cleanup from Dockerfile

Purging/cleaning packages using apt doesn't actually reduce image size, and only creates additional layers, which doesn't add any benefit to the overall image.
Merge pull request #294 from ecnahc515/patch-2

Remove apt-get cleanup from Dockerfile
actually use SshPort variable

SshPort is defined (SSH_PORT) but the value is not actually used (just compared to default port 22).
Merge pull request #335 from taziden/patch-1

actually use SshPort variable
Revert "actually use SshPort variable"
Merge pull request #338 from gogits/revert-335-patch-1

Revert "actually use SshPort variable"
update dockerfiles README.md

typo
Merge pull request #365 from choas/patch-1

update dockerfiles README.md
Merge branch 'master' of github.com:gogits/gogs into dev

Conflicts:
	dockerfiles/README.md