Set default `CMD` to `v2` in Dockerfiles if no command is set

### Feature request

Add a `CMD ["v2"]` line to `Dockerfile`s to provide a default command for serving with Docker.

### Motivation

Now that the `v2` CLI is the recommended and documented way to use `infinity`, and it fully supports configuration via environment variables, defaulting the `CMD` to `v2` can allow starting the container with no arguments but environment variables.

### Your contribution

I can submit a PR if agreements can be reached.

Start by locating the repository's Dockerfiles and checking which ones already define a CMD. Confirm how the v2 CLI is invoked and test starting the relevant image with environment variables but no command-line arguments. Done means Dockerfiles without an existing command provide the requested default while existing commands remain unchanged.

Written by the indexing model from the issue text.