We do not maintain docker.com/frrouting anymore and not building custom
images for topotests.
Use local images for topotests instead.
Just use:
```
mak topotests-build
make topotests
```
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>
We have set up automated builds for the topotest images at dockerhub.
Therefore, people don't need to perform a local build to run the
topotests on docker, but can use the image built by dockerhub.
As there is not much benefit in building locally, but the disadvantage
of weird errors if using an outdated image, change the topotests target
to always pull the most recent image from dockerhub.
Add an environment variable to disable this, e.g. for local development.
Signed-off-by: Christian Franke <chris@opensourcerouting.org>
