Sandbox docker image should run on a different port be default.

[critesjosh]

When running the docker command, the sandbox starts on port 8080 by default. This is a commonly used port and may cause people to run into error where the port is already in use.

Change the default port to something else (maybe 4273, because it looks like AZTE(C) h/t @iAmMichaelConnor).

Also consider allowing a developer to specify the port that it runs on in an environment file. An environment file may make it easier to persist changes when the command is run multiple time (so that the file storing the change isn't over-written).