Simplify GitLab CI Dockerfiles.
Consolidate some docker build instructions into distinct images for
the base dependencies, the various toolchains, and the docs build
environment. Combine Dockerfiles that only differ in toolchain version.
Total image size is prioritized over total build time by making all
apt-get layers include update, install, and cleaning in single RUN
commands, where intermediate layers are part of the tagged images.
Made images both for clang and gcc builds of the documentation.
Refs #3263
Change-Id: I55d6a128067f78214193223756db94a1964c491d
15 files changed: