Add a developer image

[osteffenrh]

Is your feature request related to a problem? Please describe.
The current images are tuned towards CI: small size, no convenience tools, etc.

Describe the solution you'd like
Add another image, based (layer on top of) on the existing ones (or separate?)
intended for development work. It could contain additional tools that are
helpful for development but not needed in the CI.

Describe alternatives you've considered
Adding the developer tools to the CI images increases the size and the
amount of data transferred in each CI run.

Questions:
Which tools should be included in the developer image?

[osteffenrh]

See

• #12

[osteffenrh]

There are some dev images now.