faster auto update images

[Evanfeenstra]

Keep the 24-hour configurable auto-update that we have now.

But add a new cron also (every 15 minutes). Hardcoded it only for stakgraph and repo2graph right now, since they both have the GET /busy endpoint. No need to add this as a config value in the swarm config

1. pull latest image. If there was a new image
2. GET /busy (returns {busy: boolean}) from the image itself
3. if busy: false then update the image
4. if busy: true then simply skip and wait for the next one in 15 minutes

[tomsmith8]

#598

[tomsmith8]

@tobi-bams could you review and also test

[tobi-bams]

@tobi-bams could you review and also test

Hi @tomsmith8, this PR is touching the auto_update cron job, and that's not what we want.

Also, some changes were made in the src/dock.rs file that are not necessary and will affect other functions.