Improves the docker build.

See merge request !11

# Conflicts:
#	Dockerfile

PNGQUANT_VERSION, GIFSICLE_VERSION, DISCOURSE_VERSION as build args, not envvars

cleanups, PNGQUANT_VERSION, GIFSICLE_VERSION moved right in front where they're used, DISCOURSE_VERSION also (better use of caching layers during build, for example when changing DISCOURSE_VERSION but not PNGQUANT_VERSION)

since they're used at build time, PNGQUANT_VERSION, GIFSICLE_VERSION, DISCOURSE_VERSION should be build args, not envvars

Support for DISCOURSE_ADDITIONAL_GEMS

support for DISCOURSE_ADDITIONAL_GEMS: properly handling bundle deployment vs. DISCOURSE_ADDITIONAL_GEMS

DISCOURSE_ADDITIONAL_PLUGINS build arg