Start daemon with Unix socket only

Mostly reproduce flow of https://github.com/docker-library/docker/blob/master/19.03/dind/dockerd-entrypoint.sh#L130
TCP socket with or without TLS isn't really needed for our usecase so
it's disable

docker/docker/Dockerfile.linux.amd64

@@ -1,4 +1,6 @@
 FROM docker:19.03.5-dind
 
+ENV DOCKER_HOST=unix:///var/run/docker.sock
+
 ADD release/linux/amd64/drone-docker /bin/
 ENTRYPOINT ["/usr/local/bin/dockerd-entrypoint.sh", "/bin/drone-docker"]

docker/docker/Dockerfile.linux.arm

@@ -1,4 +1,6 @@
 FROM arm32v6/docker:19.03.5-dind
 
+ENV DOCKER_HOST=unix:///var/run/docker.sock
+
 ADD release/linux/arm/drone-docker /bin/
 ENTRYPOINT ["/usr/local/bin/dockerd-entrypoint.sh", "/bin/drone-docker"]

docker/docker/Dockerfile.linux.arm64

@@ -1,4 +1,6 @@
 FROM arm64v8/docker:19.03.5-dind
 
+ENV DOCKER_HOST=unix:///var/run/docker.sock
+
 ADD release/linux/arm64/drone-docker /bin/
 ENTRYPOINT ["/usr/local/bin/dockerd-entrypoint.sh", "/bin/drone-docker"]