Merge pull request #558 from isayme/issue/docker-image-size

clear apt cache to reduce docker image size

Author: notheotherben

Dockerfile

@@ -8,8 +8,10 @@ LABEL org.opencontainers.image.source=https://github.com/SierraSoftworks/github-
 LABEL org.opencontainers.image.description="Backup your GitHub repositories and releases automatically"
 LABEL org.opencontainers.image.licenses=MIT
 
-RUN apt-get update && apt-get install -y \
-  openssl
+RUN apt-get update && \
+    apt-get install -y openssl && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
 
 ADD ./github-backup /usr/local/bin/github-backup