CI: change image, specify tag (fix pipeline)

The runner's allowed_images pattern does not match an image without specifying a version. Since administrator access is required in order to append or modify the runner configuration, this change attempts to satisfy the existing pattern which expects a ':' followed by a tag.

This also changes the image to run, as it turns out that the 'videolan-base-xenial' image does not have git installed which causes the pipeline to fail. The image 'vlc-debian-stable' was found to be suitable, but I have not done an exhaustive search in order to find out whether it is the optimal choice.

Following this change, the pipeline returns to normal operation which is to compile every header (i.e the result of find * -name '*.h') in the repository with both a C and a C++ compiler successfully, or fail.