Don't cache Docker layers

See merge request !6

Fix Go architecture mismatch

See merge request !5

Install apt-transport-https

See merge request !4

Newer Debian version support HTTPS for APT repositories by default, but stretch
needs this additional package. Needed to allow packages.icinga.com to
force-redirect to HTTPS.

Add go support for Strech

See merge request !3

Add Debian Bullseye

See merge request !2

Drop Debian Jessie

See merge request !1

Also set default so make build the official image names outside of CI.