Distribute as Docker image

[mircea-pavel-anton]

I think it would be helpful to also package these binaries (age and age-keygen) as Docker images.

My main usecase for this is:

• to be able to run this more easily via a simple docker run to generate a new key, for example
• to be able to pull the image and extract the binary as a "build step" in building other docker images (see here)

I already managed to package this in a docker container in this repo by cloning the git repo and running a go build, essentially.

Mainly, I was wondering if you're interested in a PR including a Dockerfile and a github workflow to build and push that docker image to ghcr.io