Docker: drop apt-get clean usage.

It's automatic in the Debian and Ubuntu containers: 5cf7949ecf/scripts/debuerreotype-minimizing-config (L85-L109)

pkg/docker/template.Dockerfile

@@ -70,7 +70,7 @@ RUN set -ex \
     && apt-get update \
     && apt-get --no-install-recommends --no-install-suggests -y install curl $(cat /requirements.apt) \
     && apt-get purge -y --auto-remove \
-    && apt-get clean && rm -rf /var/lib/apt/lists/* \
+    && rm -rf /var/lib/apt/lists/* \
     && rm -f /requirements.apt \
     && ln -sf /dev/stdout /var/log/unit.log