Dockerfile.dapper: make sure to update apt repos before fetching build deps and sources

Also, use dapper -d flag in scripts/shell.sh for better visibility

Dockerfile.dapper

@@ -39,6 +39,7 @@ RUN if [ "${TOOLCHAIN}" != "" ] && ! which ${TOOLCHAIN}-gcc; then \
     ;fi
 
 RUN if [ "${TOOLCHAIN}" != "" ]; then \
+        apt-get update && \
         cd /usr/local/src && \
         for i in libselinux libsepol pcre3 util-linux; do \
             apt-get build-dep -y $i && \