Allow turning on SSL in Docker

[interfect]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

This PR adds a feature (USE_SSL environment variable to control the Docker entrypoint) and documentation for it.

• What is the current behavior? (You can also link to an open issue here)

Currently there's no way to tell the docker entrypoint script to use SSL, so the Docker always produces a frontend that wants to talk to a "ws://" endpoint. This doesn't work behind an SSL-terminating reverse proxy.

• What is the new behavior (if this is a feature change)?

Now you can set USE_SSL=1 along with the HOST and PORT variables, to tell the Docker to serve a frontend that expects to be behind HTTPS.

• Other information:

[askmike]

👍