.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.

merged

mentioned in commit 5cb91b05