This change verifies that `docker buildx` is available and then uses that
for building the images. For that we apparently need the magic line at
the top of the `Dockerfile`s and set the environment variable
`DOCKER_BUILDKIT`. This solves a problem building docker images on
Linux with an older Docker version where the heredocs didn't work.
We need to run `apt-get update` first before trying to install packages.
We did update the package index when we built the base image, but that
might have been some time ago so that the index is now outdated.
- use KEYMAN_USE_NVM and KEYMAN_USE_EMSDK
- pre-install node (where necessary) to prevent having to do it on each
build
- adjust to current `master`
- fix a few bugs in the Dockerfile
- get required node version from package.json (through
`shellHelperFunctions.sh`)
