switch to debian:buster slim and install make

.drone.yml

@@ -18,9 +18,9 @@ steps:
     path: /var/run
   commands:
   - apt-get update && apt install -y --no-install-recommends curl make ca-certificates
-  - curl -fsSLO https://download.docker.com/linux/static/stable/x86_64/docker-${DOCKERVERSION}.tgz
-  - tar xzvf docker-${DOCKERVERSION}.tgz --strip 1 -C /usr/local/bin docker/docker
-  - rm docker-${DOCKERVERSION}.tgz
+  - curl -fsSLO https://download.docker.com/linux/static/stable/x86_64/docker-$${DOCKERVERSION}.tgz
+  - tar xzvf docker-$${DOCKERVERSION}.tgz --strip 1 -C /usr/local/bin docker/docker
+  - rm docker-$${DOCKERVERSION}.tgz
   - sleep 5 # give docker enough time to start
   - echo login $REGISTRY
   - docker login -u $USERNAME -p $PASSWORD $REGISTRY