Build and push docker images with Github Actions

[EvanKrall]

This will have Github Actions build and push docker images to toddsundsted/ktistec:latest every time you update the main branch. If you'd like, I can make it push tags to toddsundsted/ktistec:<tagname> instead / in addition.

To make this work, you'll have to:

1. create a docker hub token by going to https://hub.docker.com/settings/security
2. create DOCKERHUB_USERNAME and DOCKERHUB_TOKEN secrets at https://github.com/toddsundsted/ktistec/settings/secrets/actions

[toddsundsted]

thanks! this is great! i'm reviewing it now.

the only hold up is "shards update". i'd like dependencies to be fixed in a release—shards update changes them. it looks like, however, something happened to the backtracer shard. i'd like to address that specifically, if possible, rather just work around it with an update.

[EvanKrall]

I've removed the shards update from this branch, and also made it so it pushes to ${{ secrets.DOCKERHUB_USERNAME }}/ktistec instead of hard-coding toddsundsted/ktistec, so that forks can push to their own docker hub namespace.

[EvanKrall]

I'm realizing that my latest built docker image still doesn't have the latest code (so I'm still hitting #21), because the Dockerfile clones the dist branch (which hasn't been updated with the fix for #21.)

RUN git clone --branch ${version:-dist} --depth 1 https://github.com/toddsundsted/ktistec .

It seems to me like it would be more idiomatic to use ADD . /build/ and RUN npm run build. This would ensure that the docker image produced always reflects the version of the code that's used to build it.