GitLab

VS Code started enforcing the locale but they decided to use en_US.UTF-8. It looks like they are going to change this to C.UTF-8, which is available on every POSIX compliant system but this might take some time. So instead, we add the required en_US.UTF-8 locale to the devcontainer for now.

Otherwise, `make documentation` fails in the devcontainer.

Our VS Code devcontainer is based on our buildenv container, so we directly modify the latter.

Before this, if registry or tag got set, we would still build all images
starting from the docker hub regsitry version and latest tag.

Update to newer ns-3 version and adapt code for changes (mostly
renamed files and command line parameters) in the ns-3 submodule.