version of node-red-contrib-ttn should come from environment

[terrillmoore]

It's ok to prefer a version, but not ok to enforce that in the source code.

https://github.com/mcci-catena/docker-ttn-dashboard/blob/78498f56ef7f97f2f45cfd6c06f62fba2e08f2d7/node-red/Dockerfile#L16