Add a Docker packaging format for the Web backend

[freakboy3742]

What is the problem or limitation you are having?

The static web backend currently generates a big blob of files in a .zip, but leaves it up to the end-user how those files are served.

Docker is a common format for encapsulating a full web app in a pseudo-runnable format.

Describe the solution you'd like

briefcase package web should provide a -p docker option to generate a Docker container that can be executed to deploy the packaged app.

Describe alternatives you've considered

1. Do nothing, and leave deployment entirely up to the end user.

2. Treat this is a publication task. briefcase publish exists, but is a stub (on all platforms). "Publish to " could make more sense as a point of integration to build and publish a Docker container.

Additional context

See #2118 for a previous request of this feature.

If we choose to implement this a packaging feature, we would need to pick a web server to use in the Docker container. This is sure to be an area where there are Opinions; it also means that Briefcase puts itself on the critical path of CVEs for problems in whatever web server is selected.