Build Labgrid container images (derived from config in yocto-ktn/ci-tests)

Adding the docker files and CI config to build the Labgrid docker image. This is derived from the config in yocto-ktn with some improvements for the pipeline. The goal is to remove the Labgrid images from yocto-ktn, as they don't really belong there.

docker/client/Dockerfile

+FROM labgrid/client
+
+RUN set -e ;\
+    apt update -q=2 ;\
+    apt install -q=2 --yes --no-install-recommends rsync wget python3-pip avahi-utils libnss-mdns;\
+    apt clean ;\
+    rm -rf /var/lib/apt/lists/*;\
+    pip3 install wget
+
+COPY key/id_ecdsa* /root/.ssh/
+
+COPY key/ssh_host_ecdsa_key.pub key/ssh_host_ecdsa_key.pub
+RUN echo -n 'rpi?.local,192.168.200.? ' > /root/.ssh/known_hosts ;\
+    cat key/ssh_host_ecdsa_key.pub >> /root/.ssh/known_hosts
+
+CMD ["/bin/bash"]