Add apt update in Dockerfile, since cached lists file may be obsoleted (#149)

[docker]: Add apt update in Dockerfile, since cached lists file may be obsoleted

Author: qiluo-msft

dockers/docker-snmp/Dockerfile

@@ -17,7 +17,7 @@ RUN apt-get update && \
     dpkg_apt /deps/libsnmp30_*.deb &&                                                           \
     dpkg_apt /deps/snmp_*.deb &&                                                                \
     dpkg_apt /deps/snmpd_*.deb &&                                                               \
-    rm -rf /deps 
+    rm -rf /deps
 
 # install subagent
 RUN apt-get -y install build-essential wget libssl-dev openssl supervisor && \

dockers/docker-vas/Dockerfile

@@ -5,7 +5,8 @@ COPY user-override /etc/opt/quest/vas/user-override
 
 ## Pre-install the fundamental packages
 ## Clean up
-RUN dpkg_apt() { [ -f $1 ] && { dpkg -i $1 || apt-get -y install -f; } || return 1; } &&        \
+RUN apt-get update &&                                                                           \
+    dpkg_apt() { [ -f $1 ] && { dpkg -i $1 || apt-get -y install -f; } || return 1; } &&        \
     apt-get -y install                                                                          \
         sudo                                                                                    \
         &&                                                                                      \