Support referencing environment variables in Wireproxy config. This simplifies usingusing Wireproxy in docker-compose setups where several wireproxy containers are used to impersonate and proxy to services running elsewhere - a common config file can be used with ENDPOINT substituted for an environment variable.

I've only addressed the configuration values that were required for my setup, along with any that were relatively east to do.

I've also updated GH actions container build to include a commit sha as a tag to allow better pinning of image version.