Docker: T7568: add apt-get update as last command

Some of GitHub actions were relying on cache from apt-get update
being available.

My previous changes broke those GitHub actions.

Add `apt-get update` as one of last commands, which adds ~20Mb,
but still 2.04Gb -> 2.01Gb for vyos-build comparing with Dockerfile
without cache cleaning.

No changes for vyos image as I hope it is not used in any automatic
scripts that use `apt-get install` without prior `apt-get update`.

Author: hedrok

docker/Dockerfile

@@ -334,6 +334,9 @@ RUN rm -rf /tmp/*
 # Remove cleanup script so that in-container apt-get install uses cache
 RUN rm /etc/apt/apt.conf.d/clean
 
+# Add cache once as it is needed by some builds in GitHub Actions
+RUN apt-get update
+
 # Disable mouse in vim
 RUN printf "set mouse=\nset ttymouse=\n" > /etc/vim/vimrc.local