Update Dockerfiles (#1995)

* Revert to simple simlinks
now that image builds are triggered by cron job on CI
rather than repository links on Docker Hub

* Parallelize install/build steps using multi stages
by cascading copies of manifests and workspaces

* Rename stages for installing dependencies

* Remove redundant ARGs and fix WORKDIRs
ARGs do not need to be redeclared for child stages

* Fix typo

* Simplify ARGs and WORKDIRs

* Write package list outside of workspace
to generalize skip up to pattern

* Simplify tests
Assume current WORKDIR
Don't use build mixins for test

* Add parallel stages for testing
use target arg to build added stages, e.g:
`docker build --target=underlay_tester ...`

* Add more comments

* Add simple stage to collect test results
and to easly build all other tester stages in parallel, e.g:
`docker build  --build-arg=RUN_TESTS=True --target=workspaces_tester`