Only build Sleeper CLI Docker images against develop

[patchwork01]

Background

Split from:

• Sleeper CLI Docker images on main branch can't be pulled #2494
• Plan approach to GHCR image problem / CLI / Docker images #2684

Description

Since we're only able to retain a single tag for each Docker image in GHCR, we'd like to only push the develop version.

Analysis

We can change it to use the latest tag for the develop version. We can stop using any other tags than latest.

Ultimately we'd like to avoid putting any Sleeper code into these Docker images, but we can keep it as it is for the environment image for now as well.

Here are some places that reference the Docker images by their GitHub Container Registry image name:

• The CLI install script (?)
• The CLI script to run Docker
• The Dev Container configuration
• The GitHub Actions workflows