* docker: update build script to use glide + make

- docker/build.sh will now use glide to fetch dependencies
- glide is built from source to keep compatibility with arm
  (no pre-prebuilt binary for arm)
- docker/build.sh will also now use the provided Makefile
  It will generate an error when trying to get git build has as we do
  not ship the 88mo .git directory during the build (should not cause
  any problem as the variable it sets was not set previously)

* docker: fix docker arm build

- drop gosu version back to 1.7 as gosu binary for armhf is broken
- see tianon/gosu#19

* docker: update gosu to 1.9

Signed-off-by: Jean-Philippe Roemer <jp@roemer.im>

- 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

- 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