Add Docker Buildx for multi-arch image building.

[davidjameshowell]

This should resolve #52. I am using Docker BuildX with QEMU to cross build images for all major platforms. Node has availability on all these images, so we all amd64, arm64, arm v7, arm v6 are all supported.

I chose to build on tags, since I see you tag releases but ultimately reference the git sha of the tagged commit to ensure immutability.

Let me know your thoughts on this!

[claabs]

Looks good. I agree with the SHA tags, the releases are only to notify people if they're watching releases on Github.

The only issue I see is due to the fact that we're using an hjson package release from Github releases (https://github.com/claabs/epicgames-freegames-node/blob/master/Dockerfile#L36).
I'll need to either find it in Alpine APK, add it to Alpine APK, or parameterize the Dockerfile for arch.

Edit: or I could just use the CLI for this, since we have Node installed already: https://www.npmjs.com/package/json5

[claabs]

I just pushed the hjson fix. Here's some changes I'd like before merging.

[davidjameshowell]

Sounds good. I wasn't sure how you want to handle builds in the sense of per commit or per tag. Thought it would be a good baseline. Will update PR with effective changes.

[davidjameshowell]

The only other question I have is if you want to replace the tags username with your actual username to have it clearly displayed in the build log. Otherwise, it will asterisks it. Not a huge deal, more of a quality of life change for readability,

[claabs]

Yep, username is fine so we can eliminate the asterisks

[davidjameshowell]

Updated to hardcode username in as well as fixed the repo name on Docker Hub to what it currently is (missed that on the original review update).