fix setuptools scm and docker

[davidwyld]

Whoops, my setuptools scm introduction (#180) broke the docker build because there's no git history in there!

This is not a great fix, but it allows the container to build. I'll come up with something better shortly.

When running with the compose setup here, the _version.py in your local src will get bind mounted and the docker setup will report that as the version.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
polarrouteserver
__init__.py	3	0	100%
_version.py	13	0	100%
celery.py	15	3	0	80%
polarrouteserver/route_api
__init__.py	4	0	100%
models.py	72	2	0	97%
responses.py	33	0	100%
serializers.py	120	5	4	94%
tasks.py	124	21	7	82%
utils.py	112	10	4	91%
views.py	276	17	4	94%
polarrouteserver/route_api/management
__init__.py	0	0	100%
polarrouteserver/route_api/management/commands
__init__.py	0	0	100%
ensure_adminuser.py	22	22	0	0%
insert_mesh.py	26	26	0	0%
polarrouteserver/utils
__init__.py	0	0	100%
loggers.py	10	10	0	0%
TOTAL	830	116	85%