... and add Mint 19 and CentOS 6. The latter currently fails in
ctng's configure due to an old libtool; need to make libtool
optional.
Signed-off-by: Alexey Neyman <stilor@att.net>
Also, build containers with --no-cache: distributions like ArchLinux
retire their packages very quickly, need to always use up-to-date
package databases.
Signed-off-by: Alexey Neyman <stilor@att.net>