add github actions to build/push docker images

[chris48s]

Closes #6634

We made this switch on another repo I work on so I basically already had this knocking about.. these two actions are very slightly adapted from the solution we've used there.

In anticipation, I have set up DOCKERHUB_USERNAME and DOCKERHUB_TOKEN variables in https://github.com/badges/shields/settings/secrets/actions . I've used a token attached to my dockerhub account (which should have permission to push to shieldsio/shields on dockerhub). We could maybe look at setting up a special bot/machine account for this though if we wanted.

Interestingly the DockerHub builds for our repo seem to still be working even though they were theoretically turned off last week. I did have a look to see if Docker have announced a U-turn or extension but the latest word is that autobuilds for free plans should have been disabled on 18th June. We could just keep this PR "in the chamber" and wait to see if they actually break at some point before we make the switch. If we want to just go ahead with this, before we merge this we should probably disable the autobuilds on DockerHub so we don't have two builds trying to push to the same tags.

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[calebcartwright]

I gather GitHub still haven't added a tag create filter to this like they have for release?

[chris48s]

Yeah there isn't a specific event for tag creation. create is fired when a new ref is created. A ref can either be a tag or a branch, so the way to run an action on tag create is

on: create
jobs:
  build:
    if: github.event.ref_type == 'tag' 

[calebcartwright]

I'm inclined to go ahead and make the pivot vs. waiting to see. Maybe we could hold off till the July image publish though.

[chris48s]

Maybe we could hold off till the July image publish though.

Published ok using native dockerhub build:
https://hub.docker.com/layers/shieldsio/shields/server-2021-07-01/images/sha256-967e517c4808936a69a7d80c683be345934d39918ce4e215e58b91fbaa4e8fba

[calebcartwright]

Published ok using native dockerhub build:

Indeed. Think I'm just a bit paranoid about waiting and having the on-demand/latest builds failing without us realizing.

I don't feel strongly about it at all though so we can certainly park it for a while if you'd prefer

[chris48s]

Going to just leave this one "in the chamber" while the DockerCloud builds continue to run.
TODO: If we need to resurrect it, we could also run just the docker build action (not the push/tag) as a CI check on pull requests.

[chris48s]

Looks like DockerHub disabled our builds about a week ago so I am going to merge this.
I'll move docker build as a CI check to an issue.