Docker image update: Redis 6 and 25% smaller image size

[Ed_S]

@Falco

…we recently went over 1GB for the compressed image, and that was a bit too much.

So … we moved the Discourse source code from inside the image … to a “shallow clone” containing only the most recent version of the code.

This changes makes the compressed image size 25% smaller… It also should control the image growth over time.

Is this still working as it should be? Is there an easy one-liner to discover how large the compressed image is?