Images are missing doxygen

[Tormod Landet]

I needed to run

sudo apt update
sudo apt install doxygen

to be able to test the new docstring generation from Doxygen XML files

[Tormod Landet]

The dev-env image is now fixed, but the pipelines image must also be changed, otherwise dolfin will not build on bitbucket pipelines. The C++ build depends on doxygen, not just the Sphinx docs (due to generation of SWIG docstrings)

[Tormod Landet]

I see now that the pipelines image is not used by dolfin, only ufl etc