Docker: add kmod dependency to docker container

Docker: add kmod dependency to docker container

modprobe and rmmod are used to load the kheaders module as a last option
when kernel headers are not found. The modprobe command is missing in the
docker image and scroipts are failing:

$ /usr/share/bcc/tools/execsnoop --mntnsmap /sys/fs/bpf/mnt_ns_set
sh: 1: modprobe: not found
Unable to find kernel headers. Try rebuilding kernel with CONFIG_IKHEADERS=m (module)
...

Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>