Create pre warmed test docker image

[jfowl]

As discussed with @deepbluev7, I woul like to run the mtxclient tests against the Conduit Matrix Homeserver in Conduit's CI.

mtxclient's test job currently needs to compile the project first, which takes some time. To avoid that during Conduit's CI runs, I envision the following:

• mtxclients CI outputs a docker image (maybe just stored in the GitLab Container registry)
• In the image the source code sits at /mtxclient and has already built binaries (and maybe ccache artifacts?), so that tests can be started immediately.

Rough suggestion for how that could look like:

FROM ubuntu:18.04
RUN mkdir /mtxclient
COPY . /mtxclient/
WORKDIR /mtxclient/
RUN ... # Basically all these lines: https://nheko.im/nheko-reborn/mtxclient/-/blob/master/.gitlab-ci.yml#L54-80