Ensure we only use aptly 1.4

Change-Id: If0af112c3e0f93e7da593d20b72bbbe0ea605a10
diff --git a/mendel-builder/Dockerfile b/mendel-builder/Dockerfile
index 84e4692..2998337 100644
--- a/mendel-builder/Dockerfile
+++ b/mendel-builder/Dockerfile
@@ -21,7 +21,8 @@
 RUN /bin/bash -c ' \
 cat /tmp/aptly.pubkey |apt-key add - && \
 apt-get update && \
-apt-get install -y aptly'
+apt-get install -y aptly=1.4.0 && \
+apt-mark hold aptly'
 
 RUN /bin/bash -c '\
 curl https://storage.googleapis.com/git-repo-downloads/repo >/usr/local/bin/repo && \