Add Docker workflow

[pataquets]

• Dockerfile initial version.
• Add Docker Compose manifest.
• Readme: document usage.

I've successfully used the provided files to locally build and run the app
from source without installing Golang, libs, pip or youtube-dl.
Everything is included in the built image.

The Docker Compose manifest will take care of building the image according
the Dockerfile and later launching a container with the appropriate
settings, mainly mounting the /dev/snd device and using host networking
mode to allow SSDP to work.

Most of the PR should be self-explanatory with the provided docs.

[pataquets]

@aykevl ping

[glensc]

@aykevl please merge.

I'm building on top of this, as the submitted version used "latest" images which no longer work, so need to port to python3 and yt-dlp for example. and I prefer to use previous work than start from scratch.

[aykevl]

See #11 (comment).