docker: Switch to Debian 8

Friendlier than Ubuntu and roughly the same size.

We just need to add lsb-release to the dependencies so that yocto finds
a known distro and install gosu manually as only Debian 9 started to
package it.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Dockerfile

@@ -1,6 +1,6 @@
 # This image builds Yocto 2.2 jobs using the kas tool
 
-FROM ubuntu:16.04
+FROM debian:8.8
 
 ENV LOCALE=en_US.UTF-8
 RUN apt-get update && apt-get install -y locales && \
@@ -9,10 +9,12 @@ RUN apt-get update && apt-get install -y locales && \
 
 RUN apt-get -y install gawk wget git-core diffstat unzip \
                        texinfo gcc-multilib build-essential \
-		       chrpath socat cpio python python3 \
-		       libsdl1.2-dev xterm tar bzip2 curl \
-		       dosfstools mtools parted syslinux tree \
-		       python3-pip bc gosu python3-yaml
+                       chrpath socat cpio python python3 \
+                       libsdl1.2-dev xterm tar bzip2 curl \
+                       dosfstools mtools parted syslinux tree \
+                       python3-pip bc python3-yaml lsb-release
+RUN wget -nv -O /usr/bin/gosu "https://github.com/tianon/gosu/releases/download/1.10/gosu-amd64" && \
+    chmod +x /usr/bin/gosu
 COPY . /kas
 RUN pip3 install /kas