* fix(docker): allow the `zebra` user access to relevant dirs
When runnning a Zebra node using Docker without a privileged user, you won't be able to modify some files and directories, not even the ones in the current directory, as the `zebra` user has no permission to `/`.
The best way to solve this is making the `/opt/zebrad` the current `WORKDIR`. This also requires moving the `entrypoint.sh` from the root `/` directory to `/etc/zebrad` as this directory is used to save configuration, and other files.
An `APP_HOME` ARG is used as not all platforms where a Docker container is deployed allows writting permissions to the `/opt` directory. This allow some users to re-build the image with a custom `WORKDIR`
* fix(docker): allow starting the container without a `zebrad` command
As `gosu` is just required and available in our `runtime` image, trying to run `docker run -it --rm --name tests -t zfnd/zebra:<pr> /bin/bash` in other stages will fail, as `gosu` is not available.
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