Turns out Firefox mach does not respect `DEBIAN_FRONTEND=noninteractive`
command and insists on interactive input for the mozbuild folder
creation.
Pre-creation of the folder avoid this codepath.
This patch introduces independent docker-based build environments for WebKit and Firefox
browsers.
To launch an interactive session inside the environment:
```sh
./browser_patches/docker/cli.sh webkit-debian-11 build # builds image
./browser_patches/docker/cli.sh webkit-debian-11 enter # launches interactive container
```
