Update docker/build.sh script to use glide & make (#3079)

* 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>

.dockerignore

@@ -1,7 +1,5 @@
 .git
 .git/**
-conf
-conf/**
 packager
 packager/**
 scripts
@@ -9,13 +7,13 @@ scripts/**
 .github/
 .github/**
 config.codekit
-LICENSE
-Makefile
 .dockerignore
 *.yml
 *.md
 .bra.toml
 .editorconfig
 .gitignore
-.gopmfile
 Dockerfile*
+vendor
+vendor/**
+gogs