Add a Docker packaging format for the Web backend

### 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.

Start by reviewing #2118 and the current output of `briefcase package web`. Decide how the `-p docker` package should select and use a web server, then verify that it produces a runnable Docker container for deploying the packaged app.

Written by the indexing model from the issue text.