chore(offline): prep and make adjustments for a smoother offline experience

[lauramosher]

• Removed an unnecessary OFFLINE_MODE ENV in the prod Docker image. The default is always no, so listing it is not necessary.
• Removed SECRET_KEY_BASE from default ENV in the prod Docker image. It's a required ENV, but should be managed by the image user, rather than an empty default set that we don't update later.
• Removed asset-prefix in offline environment file: we don't compile assets using this environment, so we can share them with the default asset path.
• Added a presence check for offline mode. In the event that SECRET_KEY_BASE var exists, but is empty, we auto-generate one for them rather than throwing an error.