Replace node:lts-alpine by custom image

[piggito]

Follow up of https://github.com/skilld-labs/skilld-docker-container/pull/108/files#r346734815

1. Create custom docker image including latest node LTS + common stack (webpack, postcss)
2. Replace IMAGE_FRONT env variable to use custom docker image

[andypost]

Other way is create "caching" image for lts versions

[waspper]

Maybe some tweaks like in #189 could work?... That way it should be possible to have even multiple theme folders (for projects where for any reason, it's needed to have/use N themes). It's not 100% finished. It's just an idea for now.