Merge branch 'dd/ci-musl-libc'

A new CI job to build and run test suite on linux with musl libc
has been added.

* dd/ci-musl-libc:
  travis: build and test on Linux with musl libc and busybox
  ci/linux32: libify install-dependencies step
  ci: refactor docker runner script
  ci/linux32: parameterise command to switch arch
  ci/lib-docker: preserve required environment variables
  ci: make MAKEFLAGS available inside the Docker container in the Linux32 job

diff --cc ci/lib.sh
Simple merge