Merge latest develop into master

[benhutchins]

Lots of improvements here.

• This updates to using build args, which makes it much easier to create different version tags than using different branches or folders.
• Also pulled in work from #8 and wikimedia/mediawiki-docker#2 to use netcat for sleeping and support for SMTP.
• Update the example docker-compose.yml to support version 2 and 3 changes (removed dependency on --link shared environmental variables)