Merge branch 'dd/ci-musl-libc' into HEAD

* 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