Fix github actions tests
Fix github actions tests

Github actions are failing because the docker image used for testing is based
in Ubuntu 19.04 that is EOL now. This commit downgrades the version used for
testing to 18.04 that is LTS. It also installs the "util-linux" package that
includes the "unshare" command.

Signed-off-by: Mauricio Vásquez <95f407c1ffa7689a0a3f2f21b1bf582fe8359351@kinvolk.io>

brendangregg/bcc,tuxology/bcc,brendangregg/bcc,iovisor/bcc,brendangregg/bcc,tuxology/bcc,tuxology/bcc,tuxology/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc,tuxology/bcc,iovisor/bcc,brendangregg/bcc