add utilities and docs for debugging with docker

[davidwyld]

Finally worked out a more elegant way of doing this without interfering with the running of the compose stack in normal operations.

This makes the dockerised debugging opt-in and stops the debugger from interrupting the normal running of the django app.

As documented, opt-in and run the debugger by using the additional compose.debug.yml file with docker compose -f compose.yml -f compose.debug.yml up

The only interference with the normal docker/compose usage (which is of course only for development anyway) is that debugpy is installed by default in the app image.

[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%

[thomaszwagerman]

LGTM 🚀