Ideally instead of having names like ruby_jessie_x64_ruby_2_5 and
ruby_jessie_x64_ruby_2_6 they would have the name "ruby" with tags
containing jessie_x64_ruby_2_5/6. But fixing that would be much more
invasive. The sha1 in the tag is producing the worst effects, so this is
a case of the perfect being the enemy of the good.
Fixes#20546
With rake-compiler-dock-1.0.1 there's no need to use a derived Dockerfile.
The standard RCD image can be used instead.
The only issue is with conflicting declarations of gettimeofday(), which can be easily worked around by patching win32.h before the build.
Fixes#21514
Eric Anderson