We only need new images for bootstrapper changes
for each release. Between releases we can use debug images.
For releases we have to build images manually anyway.
Therefore, let's not build these images unnecessarily.
* remove Sunday and Monday morning runs, little value
* run test lint on main, as we do for all linters
* fixup outdated instructions
* use version hash instead of tags
* use dependabot for github actions
Signed-off-by: Fabian Kammel <fk@edgeless.systems>
