build(docker): update Dockerfile for better caching and user setup

images/actions/docker/Dockerfile

@@ -1,8 +1,10 @@
 FROM git.kjan.de/actions/runner-basic:latest
 
-RUN sudo apk add docker
+RUN sudo apk add --no-cache docker
 
-RUN sudo groupadd docker
-RUN sudo usermod -aG docker $USER
+RUN addgroup -g 999 docker
 
-RUN newgrp docker
+RUN adduser -u 1001 -G docker -s /bin/sh -D runner
+USER runner
+
+ENV DOCKER_TLS_CERTDIR=""