build: simplified and hadolintd Dockerfile

[masseybradley]

Refactors the Dockerfile build: removes mkdir commands (directories are created with WORKDIR instructions), pipes curl packages to tar, makes variables easier to override in builds using docker ARG's to set the ENV (note: might break the jx step create pr), fixes the helm3 download using the ENV variable, aliases the docker stages, consolidates and sorts and splits yum package installation on multiple lines following best practices. Dockerfile can be linted using hadolint: docker run --rm -i hadolint/hadolint < Dockerfile - directories created by make are ignored inline.