chore: publish docker image to GHCR; add helm chart

[invakid404]

This PR adds a GitHub Actions workflow which builds and publishes the Docker
image to GitHub's Docker image repository (GHCR), and adds a Helm chart for ease
of deployment in Kubernetes.

To get the GitHub Actions workflow up and running, two secrets need to be added
to the repository:

• GHCR_USERNAME: the username of the repository owner (in this case, trashhalo)
• GHCR_TOKEN: a GitHub personal access token which has the write:packages scope

The workflow runs when a new release is created and automatically builds and
tags a Docker image with both the tag name and latest.

For the Helm chart to be usable, an index.yaml file needs to be hosted
somewhere. For example, it can be hosted on GitHub Pages. Here's a
guide on how to create a Helm chart repository.

Note that the Helm chart can only set the two environment variables that are
documented in the README. I noticed that there are other environment variables
in use throughout the codebase, but I'm not familiar with them. It could be
extended in the future with those as well.

[gitpod-io]

[fr3fou]

hey, any updates on this PR? @trashhalo I've been wanting to deploy this on my own cluster for a while now

[invakid404]

Will reopen from a separate branch.