docker: prefer CMD to ENTRYPOINT

Specifying watchfrr as CMD instead of ENTRYPOINT allows one to easily
override this command when starting a docker container. This allows
simple, manual testing via (e.g.) bash. With ENTRYPOINT only the
container will simply explode with an exit code if watchfrr exits.

For instance one could start a shell session in this container via:

```
docker run --name test --rm -i -t <frr-container> bash
```

The default behavior (`docker run <frr-container>` with no command
specified) is not changed.

Signed-off-by: Wesley Coakley <wcoakley@nvidia.com>

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