CI: Use tmpfs option to Docker instead of mount.

Some tests require Asterisk to execute scripts which
are stored in /tmp. When mount is used for tmpfs there
is no ability to allow scripts to be executed from
that location.

This change switches to using tmpfs which can be told
to allow executables to be run from /tmp.

Change-Id: I0e598ca2b76af1f7f2d29f0da7b1731a214a291a