Evaluate and address review comments from Docker Hub regarding label/env

[leochr]

docker-library/official-images#14955 (comment):

LABEL, ENV should be defined at late as possible so that their changing wouldn't break unnecessary build cache. Related is the location of the RUN that installs dumb-init, it should be before the LABEL and ENV declarations to maximize the cache efficiency of the Dockerfile (i.e. installed packages/binaries that don't change because of a Open/WebSphere-liberty version bump should be defined in layers before any version-related ENV or LABEL). I believe that there are already versions that have the dumb-init install line later than the liberty version variables, so this is instead something to be considered instead of blocking this PR, but may be included if you like.