tests/topotests: Change docker build context

tests/topotests: Change docker build context

In order to support automated builds, the build context needs to be
the repository root.

So adapt our dockerfile and buildscript for this. Also, add files which
should not be included into the build context in .dockerignore to limit
the size of the build context to something sensible.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>