Auto merge of #1297 - mati865:docker-images-working, r=gnzlbg

Upgrade bunch of EOL docker images to Ubuntu 18.04

When upgrading other Dockerfiles (not modified in this PR) they fail with different errors:
https://travis-ci.com/mati865/libc/jobs/181623075
https://travis-ci.com/mati865/libc/jobs/181623059
https://travis-ci.com/mati865/libc/jobs/181623067

I think it's because of these 2 commits in glibc:
https://sourceware.org/git/?p=glibc.git;a=commit;h=b05cb613e7ae507680c8d914b3e788eea782c0d6
https://sourceware.org/git/?p=glibc.git;a=commit;h=2dba5ce7b8115d6a2789bf279892263621088e74

I have no idea how to fix them so leaving it for somebody who knows what to do.