Align Name/Version scheme with Community Standards

[drGrove]

Normally when you have a package that has multiple distros, versions and/or build platforms you continue to have the same top level repository and just modify the version. This allows for easier switching of the images strictly on a version basis as well as a single repository to see all of the tags. See Python

For this repo you'd be looking at something like:
asbjornenge/tezos:alphanet-{alpine,ubuntu}
asbjornenge/tezos:zeronet-{alpine,ubuntu}
asbjornenge/tezos:mainnet-{alpine,ubuntu}

Your arm based images can also push to the same repository as dockerhub breaks them up automatically for you using Digests. Which ensures that a user is being more specific with what version they are running.