Docker image: add `major.minor` tags

[pataquets]

First and foremost, thanks for sharing such a useful piece of software.
Currently, only major.minor.patch version tags are available. You can pin the exact version (e.g. :0.16.2). So far, so good.
However, if you want to, let's say, stay up to date with all 0.16.x patch versions as they are released, there is no :0.16 (no patch) tag, which would always point to latest :0.16.x tag.
This would strike a nice and convenient balance between pinning versions (ie. no :latest) and stay up-to-date on fixes.
See Golang's public Docker image as example.

[sosedoff]

Thanks for the suggestions @pataquets