Addressed some security issues in Dockerfiles.

Summary:
- Removed --trust-server-cert from `svn checkout` invocations.
  Installing 'ca-certificates' package on ubuntu adds required CAs to
  the system and svn can do proper checkout using https.

- Added checksum verification when installing cmake from cmake.org.

Reviewers: mehdi_amini, klimek

Reviewed By: mehdi_amini

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36673

llvm-svn: 311152