Activate lightwalletd RPCs in Docker image when RPC port is set

[teor2345]

Motivation

The runtime-entrypoint.sh uses the RPC_PORT env var when the user
specifies the getblocktemplate-rpc feature

Some users might want to use the Docker image with lightwalletd, but without configuring getblocktemplate-rpcs. So let's work out how to do that!

Previously, the Dockerfile only activated the RPC port when it was set, but I agree that doesn't make sense with getblocktemplate-rpcs:
8c90f65#diff-5b838857fc3410c2438196effdbdcffae6548bdba431a4a938e76a0be62d008eL49

So we might want logic like:

• If getblocktemplate-rpcs is activated, activate the default RPC port if it is unset
• If there aren't any active RPC features, activate the RPC port only if it is set
• Otherwise, don't activate the RPC port

This covers the 3 different use cases of mining, light wallet, and no RPCs.

Originally posted by @teor2345 in #7162 (review)

[teor2345]

We could also have another env var that activates the RPC port the same way that the getblocktemplate feature does. (Just with the default ports.)