Fix GitHub action for docker build

[stweil]

• Remove the time consuming calculation of used disk space.
• Remove all container images (more robust than removing single images).

[stweil]

@kba, please include this PR also in the next release.

[bertsky]

362 had better solutions. (And you cannot remove all docker images, as you will need the currently relevant Ubuntu versions.)

[stweil]

Removing all docker images would have been the simplest solution. And it works because the required Ubuntu image (and only that one) will simply be installed when required. That does not take much time.

[bertsky]

Removing all docker images would have been the simplest solution. And it works because

It does not work. I have tried that elsewhere. Also, as the names keep changing, opt-in is not predictable.