fix(docker): add default CMD to test stage in Dockerfile (#9334)

* fix(docker): set `HOME` environment variable explicitly in Dockerfile

The HOME environment variable was defaulting to /root when the container started, causing cache directories to be incorrectly set up under /root/.cache/zebra instead of /home/zebra/.cache/zebra. This explicit setting ensures the HOME environment variable is correctly set to the zebra user's home directory.

* fix(docker): add default `CMD` to test stage in Dockerfile

When running the container using docker-compose without explicitly
providing a command, the entrypoint.sh script was attempting to execute
`exec_as_user` with no arguments, resulting in a gosu error:

```
exec_as_user
exec gosu 10001:10001
Usage: gosu user-spec command [args]
```

By adding `CMD ["cargo", "test"]` to the test stage in the Dockerfile,
we ensure a default command is available for the entrypoint script to
execute, preventing the gosu error when no command is explicitly provided.

This fix allows `docker-compose.test.yml` to run successfully without
needing to specify a command in the service definition.

docker/Dockerfile

@@ -133,6 +133,7 @@ COPY --chown=${UID}:${GID} ./ ${HOME}
 COPY --chown=${UID}:${GID} ./docker/entrypoint.sh /usr/local/bin/entrypoint.sh
 
 ENTRYPOINT [ "entrypoint.sh", "test" ]
+CMD [ "cargo", "test" ]
 
 # This stage builds the zebrad release binary.
 #