Switch to node slim images for 80% smaller container

[jcotton42]

Don't merge this just yet, there's some additional tweaks I want to make to the Dockerfile, like setting NODE_ENV in the appropriate stages instead of relying on them being set in the Compose file.

It's all ready to go!

Things left to do:

• [x] Set NODE_ENV in Dockerfile
• [x] Look into whether npm run prod properly handles signals. It doesn't seem to https://github.com/nodejs/docker-node/blob/main/docs/BestPractices.md#cmd and node itself may need a wrapper init process https://github.com/nodejs/docker-node/blob/main/docs/BestPractices.md#handling-kernel-signals. It appears that the latter can be set in the compose and swarm definitions https://docs.docker.com/compose/compose-file/#init.