Add docker support

[papodaca]

Adds Dockerfile and docker-compose.yml

Some potential improvements:

• Add heath check command to docker file
• Add entrypoint to generate config file at runtime from environment variables

Closes #2

[frou]

Thanks for this. Some rudimentary questions:

• I'm aware that there's an Alpine community package for youtube-dl. Do you reckon it's better to use pip regardless?

• Is the purpose of having the 2 separate FROM stages to stop the container being bloated with the Go toolchain?

• It seems like it would be impractical to convey the application config using environment variables because there can be an arbitrary number of podcasts. Is the way that you've mapped the config file in from the Docker host the generally accepted way to do things?

[papodaca]

• AFAIK the latest version of youtube-dl is always punished to Pypi the alpine package maintainer may not be taking the latest version.

• Multistage builds are used for exactly this reason. More info.

• Its typical that for things like port number this information comes from environment variables. You are right that in this case though it might not make sense.