chore: docker releases

[kiwicopple]

- Summary

Adds an automated docker release on every push to master. Closes #179

- Test plan

Add secrets (see below), commit to master, check docker hub.

- Description for the changelog

This change adds an action to release to docker hub every time there is a commit to the master branch. This is a small improvement - it simply adds a docker image with the tag dev (i.e: netlify/gotrue:dev). I chose this tagging so there isn't any ambiguity of production support, but it makes it easy for contributors and developers to get started using gotrue

It requires 3 secrets to be added here: https://github.com/netlify/gotrue/settings/secrets

DOCKER_REPO              # netlify/gotrue
DOCKER_USERNAME    # someone with access
DOCKER_PASSWORD    # the user's password