dockerhub build issues

[jchodera]

For the record, the recent dockerhub build failures were addressed with this workaround.

[jchodera]

I've implemented the workaround, but now I am running into build timeout issues. It's apparently not possible to completely build the X86 target for clang in the 2 hour build window. Trying to separate just the clang build into a separate image layer didn't help.

My current thinking is that we can either:

• (1) Possibly build the image separately and upload it (undesirable for maintenance purposes)
• (2) Break the build into separate build substeps in separate RUN commands that will iteratively build up different layers that are cached with separate build triggers. The build emits a series of target build notifications, like:

Scanning dependencies of target LLVMSupport
...
Scanning dependencies of target LLVMTableGen
...
Scanning dependencies of target obj.llvm-tblgen
...

I wonder if this means we can have make build a few these in separate RUN statements.