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

[stevapple]

Feature request

Add a CMD ["v2"] line to Dockerfiles 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.

[michaelfeil]

@stevapple I just removed the default to v1. Such changes are hard to revert / change your mind about. If this issue becomes popular, I might consider it.