add `--workdir` to `docker run` only for `dmake shell` (#302)
add `--workdir` to `docker run` only for `dmake shell` (#302)

DMake executes services in 2 modes:
- dev (`dmake shell`)
- run (`dmake run`, `dmake test`)

In run mode:
- service v1: `WORKDIR` is always set in the dmake-generated Dockerfile
- service v2: we *do not* want --workdir there: it's the
  responsibility of the user-provided Dockerfile

=> we need workdir only for dev mode.
Deepomatic/dmake,Deepomatic/dmake,Deepomatic/dmake,Deepomatic/dmake