Make EPICS CA PORTS and the network subnet configurable

[gilesknap]

Intro

This allows a user to set the range of ports and the network subnet that is used by the compose project.

Benefits

• We no longer tie up the default channel access ports so developer containers can use them instead
• We could potentially run up multiple of these compose projects on a single host
• The changes also incorporate PVA so that the AD PVA plugin is now used in the example project
• It's really easy to configure by changing BASE in environment.sh see
  https://github.com/epics-containers/example-services/blob/configurable-net/environment.sh#L21-L33

Downsides

there is more setup in environment.sh and perhaps it is less legible as a result EDIT: this downside is fixed by #18

[gilesknap]

This is a refresh of the example-services project that makes it fully configurable so that it can use unique PORTS and container network subnet.

It's made things a tiny bit more convoluted and has added a few lines of funky bash to environment.sh. Including bash arithmetic evaluation.

@coretl @DiamondJoseph your choices are:-

• abandon this as too obtuse and leave it in a branch
• merge it into main and keep it as an example only
• merge it and apply the same changes to the services-template-compose copier template

The final choice means that all future compose beamlines would look similar, including those made in the tutorials.

[gilesknap]

Why am I hesitant about this change?

Because it feels a little off that there is code outside of the compose.yml files to make it work.

Up until now the only reason for environment.sh was to set a UID:GID for starting phoebus and that was just because of podman/docker differences.

Now I have extended the need for environment.sh and it is adding a prior templating step via sed. This is required because pvagw and phoebus cannot be configured by environment variables as far as I can tell.

I could put that templating step inside the phoebus and pvagw containers but that feels more unwieldy (because each time I think of a new thing to make configurable I need to rebuild the containers)

UPDATE: I have removed all of the additions to environment.sh and used a .env file plus an init container. I think this looks neater. See #18