docker images could use multi-stage builds

[marosivanco]

There is a lot going on in the httpd Dockerfile: install build/runtime dependencies, download sources, build httpd...
The images could benefit from multi stage builds (available from Docker 17.05).
The basic idea is to separate the whole process into two stages:
Build - which can create temp image with build-time dependencies, download sources, check sum, build.

(unnamed) - which installs run-time dependencies and copies built artefacts from the Build stage.

[tianon]

docker-library/official-images#3383