.gitlab-ci.yml: Allow building under docker
- use debian unstable docker image
- use block scalar indicator to avoid awkward quoting
- mount binfmt_misc
- make installing dependencies more explicit
With these changes, the reform-system-image gitlab CI pipeline can run in a gitlab docker runner:
- salsa.debian.org: https://salsa.debian.org/josch/reform-system-image/-/jobs/3293269
- gitlab.com: https://gitlab.com/josch/reform-system-image/-/jobs/3081330296
Using docker would allow us to enable CI for more repositories and allow for CI runs for each merge request.