Add production Dockerfile for deployment.

[spwoodcock]

PR #458 was rejected, as it impacts current CI processes.

This Dockerfile is an alternative to use with production deployments.

Features:

• Update Node 8 --> 14.
• Multi-stage for image separation and size reduction.
• Includes any modified packages in the build (e.g. ./packages/terriajs).
• Contains ARGs for passing coporate proxy variables, if required.

Notes:

• Based on official node:14-buster-slim, which is ~20mb larger than the Alpine equivalent, but allows for easier inclusion of GDAL.
• I would suggest adding a .dockerignore to the base repo, but don't want to impact any existing CI or workflows. Something like:

**npm-debug.log
**/.vs
**/.vscode
**launch.json
**.yml
**.yaml
**Dockerfile
**.dockerignore
**.gitignore
**.env
**.editorconfig
**.eslintrc
**.gitattributes
**.gitignore
**.gitmodules
/node_modules
/architecture
/cesium-terrain-server
/ckanext-cesiumpreview
/deploy
/doc