Merge branch 'improve-docker' into 'master'

docker: enable building selected version/tags

See merge request !21

docker/README.md

+Docker Build
+------------
+
+```
+$ export SHOTGUN_VERSION=v20200914
+$ docker build --no-cache -t registry.nic.cz/knot/shotgun:$SHOTGUN_VERSION --build-arg SHOTGUN_VERSION=$SHOTGUN_VERSION shotgun
+$ docker build --no-cache -t registry.nic.cz/knot/shotgun/pellet:$SHOTGUN_VERSION --build-arg SHOTGUN_VERSION=$SHOTGUN_VERSION pellet
+
+$ docker login registry.nic.cz
+$ docker push registry.nic.cz/knot/shotgun:$SHOTGUN_VERSION
+$ docker push registry.nic.cz/knot/shotgun/pellet:$SHOTGUN_VERSION
+```

docker/pellet/Dockerfile

 FROM ubuntu:focal
+ARG SHOTGUN_VERSION=v20200914
 
 ENV DEBIAN_FRONTEND=noninteractive
 
@@ -12,6 +13,6 @@ RUN \
 RUN pip3 install dnspython
 RUN pip3 install git+git://github.com/kbandla/dpkt.git@2c6aada35c38c93061ac6d4c47862174ffcc4799
 
-RUN git clone https://gitlab.labs.nic.cz/knot/shotgun.git
+RUN git clone -b $SHOTGUN_VERSION --depth=1 https://gitlab.labs.nic.cz/knot/shotgun.git
 
 ENTRYPOINT ["shotgun/pellet.py"]

docker/shotgun/Dockerfile

 FROM ubuntu:focal
+ARG SHOTGUN_VERSION=v20200914
 
 ENV DEBIAN_FRONTEND=noninteractive
 
@@ -30,6 +31,7 @@ RUN \
 	cd ..
 
 RUN \
-	git clone https://gitlab.labs.nic.cz/knot/shotgun.git
+	git clone -b $SHOTGUN_VERSION --depth=1 \
+	https://gitlab.labs.nic.cz/knot/shotgun.git
 
 ENTRYPOINT ["dnsjit/src/dnsjit", "shotgun/shotgun.lua"]