Do not run yum update during docker build

Dockerfile

@@ -12,8 +12,7 @@ ADD docker/mysql.repo /etc/yum.repos.d/mysql.repo
 # This is temporary until https://bugzilla.mozilla.org/show_bug.cgi?id=1226533
 ADD docker/nodesource.repo /etc/yum.repos.d/nodesource.repo
 
-RUN yum update -y \
-    && yum install -y \
+RUN yum install -y \
         # Supervisor is being used to start and keep our services running
         supervisor \
         # General (dev-) dependencies

Dockerfile.deploy

@@ -12,8 +12,7 @@ ADD docker/nodesource.gpg.key /etc/pki/rpm-gpg/RPM-GPG-KEY-nodesource
 ADD docker/epel.gpg.key /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
 ADD docker/nodesource.repo docker/epel.repo /etc/yum.repos.d/
 
-RUN yum update -y                                           \
-    && yum install -y                                       \
+RUN yum install -y                                          \
         gcc-c++                                             \
         gettext                                             \
         # Git, because we're using git-checkout dependencies