Dockerfile: apt-get upgrade -y to get the latest packages

[Al2Klimov]

regardless of the image being up to date or not.

• CC https://github.com/Icinga/docker-icinga2/pull/133

[Al2Klimov]

I'd like you to merge this, given that there's a complete v2.12.2 milestone already.

[julianbrost]

@Al2Klimov requested a review from @julianbrost 1 hour ago @Al2Klimov assigned @julianbrost 9 minutes ago

Such impatience!