Add Docker Image

[mkuf]

Hi,

I'd like to run the mobileraker_companion in Docker to be able to add it to my project: https://github.com/mkuf/prind

This PR adds:

• A Dockerfile at the Root of the repository, along with a dockerignore file
• A github workflow that builds the docker image and uploads it to ghcr.io
• A few lines of documentation about the image in the main Readme

Commits to the main Branch will build the latest tag for the Image.
Tagged commits will result in an image tag equal to the git tag.

For the Workflow to function, you'll have to enable Read and Write Permissions at Settings/Actions/General/Workflow Permissions, otherwise the Image upload to ghcr will fail.

Looking forward to your response.
-Markus

[Clon1998]

Hey,
thanks for your contribution! I will look into that hopefully later today or tomorrow. In case I forget about it, just ping me again.

Also your PrintD project looks really interesting and I am proud you want to include mobileraker to it.