use .env and init container

[gilesknap]

tidy up the configurable version of example-services by using a .env file and init container.

With the addition of a default value for UIDGID we now have the project fully described in compose yaml. The environment.sh script is a convenience but at DLS you can launch the project without that file using these commands:

module load docker-compose
docker compose --profile test up -d

This achieves my goal of making this a "pure" docker compose implementation.

[gilesknap]

YES.

One side affect of this is that compose is clever enough for this:-

dc up -d
# edit .env and change some of the EPICS ports
dc up -d 
# compose restarts ONLY the services that need it and everything comes to life with the new ports

e.g. if I change PVA server port the ca-gateway is left alone. This paves the way for tuning the behaviour by limiting which environment variables are passed to each container (e.g. we could keep the CA port inside the container network always the same)