[dockerfile] use glide and small improvements

[bamarni]

Follow-up of #46 to fix the dockerfile build too. It also :

• uses an explicit tag for alpine instead of latest
• doesn't expose build-time specific env vars
• removes unneeded build deps

Btw, is there an official image on docker hub?

[deepthawtz]

No public docker images are uploaded anywhere.

Thanks for fixing the Dockerfile as I left it broken 😄 .