Minimize docker image (continuation of #4730)

[th3w4y]

Short Description:

Fixes some issues from PR #4730

Fixes

• busybox tar does not support --strip-components argument - added and removed regular tar in same RUN step
• rectify cache invalidation issue in Dockerfile after merge of Minimize docker image #4730 (because the download of the $BUILD_BRANCH.tar.gz was in a an RUN step and cache was not invalidated - no new image would have been built)
• split pgoencrypt part into a different layer (this is in order to check if there is a new pgoencrypt version ) - executed in an different step after requirements to not cause the invalidation of the requirements build step (which is the most expensive)

[mention-bot]

@th3w4y, thanks for your PR! By analyzing the annotation information on this pull request, we identified @walkerlee, @nivong and @lawliet89 to be potential reviewers