Include a version file within the docker images to easily identify what git repo and tag was used for the image

[ljdelight]

After an image is built, it's not super easy to determine what it was based on. It is helpful to easily identify the source version without depending on the image tags.

It could look like this:

# cat /.version
repo=git:maproulette/maproulette2
tag=release/v4.2.0