Merge pull request #8458 from kfei/docker/reduce-image-size

Reduce the size of Docker image

docker/Dockerfile

@@ -2,10 +2,10 @@ FROM ubuntu:14.04
 
 # Install required packages
 RUN apt-get update -q \
-    && DEBIAN_FRONTEND=noninteractive apt-get install -qy \
+    && DEBIAN_FRONTEND=noninteractive apt-get install -qy --no-install-recommends \
+      ca-certificates \
       openssh-server \
-      wget \
-    && apt-get clean
+      wget
 
 # Download & Install GitLab
 # If the Omnibus package version below is outdated please contribute a merge request to update it.