add openssh-server into packages.txt for gitlab-shell to work in GDK
requested to merge chiehmin18/gitlab-development-kit:add-openssh-server-into-package-list into master
developing and testing gitlab-shell need a working sshd. this patch add openssh-server into package.txt which will be installed into docker image to provide sshd.
Signed-off-by: Chieh-Min Wang chiehminw@synology.com