Fix docker image builds

[lewisgoddard]

Fixes #3947

The dev Docker environment was broken due to several issues that prevented the container from building and serving the site.

Changes Summary

• Replaced the broken NodeSource node_18.x apt repository (EOL since April 2025, no longer distributing packages) with a multi-stage build using the official node:18 image
• node, npm, and npx are now copied directly from the official image; npm/npx are created as symlinks into node_modules/npm/bin/ to preserve correct module resolution
• Consolidated into a single apt-get layer, removing the now-unnecessary NodeSource setup steps
• Fixed composer up → composer install (composer up is not a valid command)
• Added set -e so the script exits immediately on any error rather than silently continuing through failures
• Removed version from dev/docker-compose.yml to get rid of a warning
• Removed submodule from normal builds as image cache makes this not required

Testing

Run cd ./dev && docker-compose up and wait for the build to finish, then load localhost:8000

This pull request is ready for review.

[tintou]

looks good!