Set default ENTRYPOINT for distroless Dockerfiles on Mariner

[mthalman]

In our normal (non-distroless) Dockerfiles, we don't use an ENTRYPOINT instruction in the runtime/aspnet/sdk Dockerfiles to allow downstream consumers to easily decide which command/executable they want. But for distroless Dockerfiles, there's literally only one executable in the whole container: dotnet. So we may as well set that as the default:

ENTRYPOINT ["/usr/bin/dotnet"]
CMD ["--info"]

In that case, the proposal would be to set the default command with the --info option.

[mthalman]

We'll apply this starting with .NET 7.