Verify static files in docker container

[KevinMind]

Relates to: mozilla/addons#2000

Description

This PR introduces checks for static files in our docker container. I achieve this with 3 changes:

• extracting the logic for copying static assets from node_modules into a shell script
• adding a check in the shell script to verify the assets are in the correct location in the file system
• allowing the script to execute in a --check-only mode (useful for CI) where we rely on the docker build to copy and only use the check to verify the work
• setting the NODE_PATH environment variable in the docker build so it can be sed across makefile scripts and executables.

Context

The script expects the NODE_PATH environment variable to be set so if you run outside a docker container, you have to set it and likely you don't have the correct node_modules installed so it should ideally only be run via the make command.

Testing

You can run the command locally via

make update_assets

to copy files and

./scripts/copy_node_assets.sh --check-only

to verify the assets.

[KevinMind]

This is totally unnecessary as the docker build actually fails when there are any missing but expected npm static file dependencies.

the compress_assets command imports specific files declared in our MINIFY_BUNDLES map, if any file is missing, it will raise a not found error.