fix typo in README for Docker

Fix bug introduced in #1759 preventing ssh server to launch

- ssh/setup: test directive now check if a file exist in key path instead of a folder
- ssh/setup script was hanging waiting for an input about rewriting the key
  as the test case was returning true, when it should have been false
  (check if file is a folder instead of a file)

Docker repository path backward compatibility fix

- resolve #1765
- create link to old git repository path: `/home/git/gogs-repository`

Volumed data setup changed to allow #1759

- Volumed subfolder now created up in the ENTRYPOINT script, this way
  they are created before S6 even starts making VOLUME.
- The subfolder will be created during VOLUME creation too as ENTRYPOINT
  script will be run before /bin/true
- SSH Keys will now be created on a single key basis not replying on the
  existence of /data/ssh folder

Docker Container Restart Fix

- Fix s6 fifodir error on container restart
- Add .tags* to .gitignore (Atom auto ctags generation)

Update gosu, this fixes #1756

fixed dead links in README

Added repository remove option to API

Fix #1669 on develop

Add links to forks/watchers/stars in new layout header

Docker Container: Init 1 & Initialisation

- Now using a setup script before starting the app. The separation of
the run script and the setup script will make service initialisation a
little bit clearer
- Now calling start.sh script as ENTRYPOINT and S6 as CMD. This way
when running the container with just a shell script, the start.sh
script will be launched before, making debugging easier
- Added note about `.dockerignore` ignored during Docker Hub Automated
Build

Several bugfixes in Docker build

- Removed unnecessary variables
- Fixed symbolic links creation
- Fixed enter point
- Less intermediate containers

New approach to Gogs Docker Container

- VOLUME for ‘/data’
- Usage of S6 as PID 1 Process
- Usage of ‘socat’ so linked container (like databases) are binded to
localhost
- OpenSSH, Socat Link and Gogs are supervised using S6
- Size of container reduced to ~75Mo

Implemented #1721: see users who forked/starred/watched a repository