Move detection of existing images in base image to own web service

[nuest]

Right now as implemented in

the filtering of base images relies on the containerit container to have access to a Docker socket (works) but also the container must be run as a user with access to Docker - which the rstudio user is not. One could add rstudio to the local Docker user group on developer machines, but that's not good.

So the package detection only works when muncher is running containerised.

A better solution in the long run would be to move the detection of installed packages to it's own API endpoint, whose implementing service could be written in R (or just be added to o2r-inspecter).

A web service would also be beneficial for caching!

[nuest]

Would make things too complicated, let's leave this within containerit.