automation: add openSUSE Tumbleweed CI image

automation: add openSUSE Tumbleweed CI image

openSUSE comes in two flavours: Leap, which is non-rolling, and released
annualy, and Tumbleweed, which is rolling.

Reasons why it makes sense to have both (despite both being openSUSE,
package lists in dockerfiles being quite similar, etc) are:
- Leap share a lot with SUSE Linux Enterprise. So, regressions on Leap,
  not only means regressions for all openSUSE Leap users, but also helps
  prevent/catch regressions on SLE;
- Tumbleweed often has the most bleeding-edge software, so it will help
  us prevent/catch regressions with newly released versions of
  libraries, compilers, etc (e.g., at the time of writing this commit,
  some build issues, with GCC9, where discovered while trying to build
  in a Tumbleweed image).

Note that, considering the rolling nature of Tumbleweed, the container
would need to be rebuilt (e.g., periodically), even if the docker file
does not change.

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Acked-by: Doug Goldstein <cardoe@cardoe.com>