Improve node version image handling

To improve node version image handling:

Remove version-specific branches
Update DOCKERFILE to use ARG for image tag. Set default to lts. Update build script to pass build-arg from environment variable.
Add dynamic child pipelines for each applicable node release with applicable environment variable

Edited Oct 23, 2020 by Aaron Goldenthal

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information