It gives you one image per configuration, instead of one build machine, or one stored VM. It makes it easier for a single machine to iterate over dozens or hundreds of build environments with a minimum of overhead.
For example, one Ubuntu machine with Docker can create build environments for every Linux based distro (and version thereof) that exists, and iterate through all of them when it comes time to build. The setup is not trivial, though it is easier than creating a similar environment with VMs or physical machines.
Every distribution on the planet handles this with chroots for doing builds and it works out just fine without needing to do gymnastics with docker. Fedora, for example, has a farm of build hosts that are architecture specific (so there is no cross-compiling outside of bootstrapping a new build target), from there a new chroot gets created for every build and the build runs isolated within it. They've been using this same system to maintain packages for up to 7 different releases at once (counting the EPEL repositories) with no issues, I fail to see any benefit docker brings to the table here - and really, nginx should just provide a stable ABI.
For example, one Ubuntu machine with Docker can create build environments for every Linux based distro (and version thereof) that exists, and iterate through all of them when it comes time to build. The setup is not trivial, though it is easier than creating a similar environment with VMs or physical machines.