build-docker.sh does not assume PWD=<repo root> (#302)
build-docker.sh does not assume PWD=<repo root> (#302)

* bash variables in build-docker.sh are wrapped by curly brackets

* Ensure presence of config file while running build-docker.sh

* Do not assume that build-docker.sh is run from the repository root directory

* Mount config file in predictable location in docker container

jacen92/pi-gen,RPi-Distro/pi-gen,WLANThermo/pi-gen