Add PVA and improve configurability

[gilesknap]

Standardize docker/podman Checks

re-using code snippets from the latest ioc-template for testing which of podman / docker we should use in scripts. The previous approach of using alias in a bash script was flawed.

Make Ports and Networks Configurable

This allows a user to set the range of ports and the network subnet that is used by the compose project. The configuration is done via a .env in the root and changes are propogated to configuration files using an init container.

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

Add PVA support

Add in pvagw and switch to using the PVA plugin for displaying simulation detector data.

[gilesknap]

@DiamondJoseph I'm aware you are basing some work on this repo right now so thought I'd alert you to some PRs I"m doing. This one is pretty benign but the other one might be more contentious.