Install apt-transport-https

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.

stretch/x86/Dockerfile

@@ -7,7 +7,7 @@ RUN apt-get update \
 
 RUN apt-get install -y \
   sudo wget curl tar expect git \
-  devscripts build-essential lintian ccache pbuilder aptitude apt-utils
+  devscripts build-essential lintian ccache pbuilder aptitude apt-utils apt-transport-https
 
 RUN echo "deb http://deb.debian.org/debian stretch-backports main" >/etc/apt/sources.list.d/backports.list
 

stretch/x86_64/Dockerfile

@@ -7,7 +7,7 @@ RUN apt-get update \
 
 RUN apt-get install -y \
   sudo wget curl tar expect git \
-  devscripts build-essential lintian ccache pbuilder aptitude apt-utils
+  devscripts build-essential lintian ccache pbuilder aptitude apt-utils apt-transport-https
 
 RUN echo "deb http://deb.debian.org/debian stretch-backports main" >/etc/apt/sources.list.d/backports.list