docker: remove git-objects from final docker image

Removing git-objects, which are not needed, from final docker image, to make it
smaller. Some files still need to be there, because Etherpad at startup uses
them to discover its version number.

.dockerignore

@@ -1,3 +1,22 @@
 .hg
+
+# Remove the git objects, logs, etc. to make final image smaller.
+# Some files still need to be in the .git directory, because Etherpad at
+# startup uses them to discover its version number.
+.git/branches
+.git/COMMIT_EDITMSG
+.git/config
+.git/description
+.git/FETCH_HEAD
+.git/hooks
+.git/index
+.git/info
+.git/logs
+.git/objects
+.git/ORIG_HEAD
+.git/packed-refs
+.git/refs/remotes/
+.gitignore
+
 settings.json
 src/node_modules