tests: allow to load kernel modules from a test container

CRIU needs to load a few modules to checkpoint/resume containers.

https://github.com/opencontainers/runc/issues/1745
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

Dockerfile

@@ -20,6 +20,7 @@ RUN apt-get update && apt-get install -y \
     protobuf-compiler \
     python-minimal \
     uidmap \
+    kmod \
     --no-install-recommends \
     && apt-get clean