Document mounting the workdir

[poikilotherm]

For the action: the working directory from the runner will be mounted at WORKDIR (https://docs.github.com/en/actions/creating-actions/dockerfile-support-for-github-actions#workdir)

For anyone else: mount to WORKDIR (/usr/src/app)

When HERMES workflow understands workdir configs from env vars, add an env var to the container to default to this place. Until then will require manual configuration.