Make backend read whether it's in development mode from the environment

[lberrymage]

Our backend currently reads whether it's in development mode via Java system properties. However, because the backend prefixes its cookies with "__Host-" in production mode and the official container image is always in production mode, it is difficult if not impossible to fully run and test the official container image locally.

We should instead read this configuration from the environment as we do with all other configuration to facilitate running the official container image locally for testing purposes.