Build container: Base build image on Debian Stretch (#24967)

* scripts/build/ci-build: Fix Makefile
* scripts/build/ci-build: Base Docker image on Debian Stretch
* scripts/build/ci-build: Modify Dockerfile for fewer RUN directives

scripts/build/ci-build/Dockerfile

-FROM ubuntu:20.04 as toolchain
+# Use old Debian (this has support into 2022) in order to ensure binary compatibility with older glibc's.
+FROM debian:stretch-20200514 AS toolchain
 
 ENV OSX_SDK_URL=https://s3.dockerproject.org/darwin/v2 \
     OSX_SDK=MacOSX10.10.sdk \
@@ -71,8 +72,9 @@ RUN cd /tmp && \
     rm -rf /tmp/x86_64-centos6-linux-gnu/ && \
     rm -rf /tmp/crosstool-ng-${CTNG}
 
-# base image to crossbuild grafana
-FROM ubuntu:20.04
+# Base image to crossbuild grafana.
+# Use old Debian (this has support into 2022) in order to ensure binary compatibility with older glibc's.
+FROM debian:stretch-20200514
 
 ENV GOVERSION=1.14.1 \
     PATH=/usr/local/go/bin:$PATH \
@@ -86,7 +88,7 @@ COPY --from=toolchain /tmp/x86_64-centos6-linux-gnu.tar.xz /tmp/osxcross.tar.xz
 RUN apt-get update && \
     apt-get upgrade -yq && \
     apt-get install -yq  \
-        clang gcc-aarch64-linux-gnu gcc-arm-linux-gnueabihf gcc-mingw-w64-x86-64 \
+        build-essential clang gcc-aarch64-linux-gnu gcc-arm-linux-gnueabihf gcc-mingw-w64-x86-64 \
         apt-transport-https \
         ca-certificates \
         curl            \
@@ -99,7 +101,12 @@ RUN apt-get update && \
         xz-utils        \
         expect          \
         gnupg2          \
+        procps          \
+        ruby            \
+        ruby-dev        \
+        rubygems        \
         unzip &&        \
+    gem install -N fpm && \
     ln -s /usr/bin/llvm-dsymutil-6.0 /usr/bin/dsymutil                  && \
     curl -fL https://nodejs.org/dist/v${NODEVERSION}/node-v${NODEVERSION}-linux-x64.tar.xz \
       | tar -xJ --strip-components=1 -C /usr/local                      && \
@@ -130,9 +137,4 @@ RUN cd /tmp && \
     tar xf x86_64-linux-musl-cross.tgz && \
     rm x86_64-linux-musl-cross.tgz
 
-RUN apt-get install -yq gcc libc-dev make && \
-    gpg2 --keyserver hkp://keys.gnupg.net --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3 7D2BAF1CF37B13E2069D6956105BD0E739499BDB && \
-    curl -fsSL https://get.rvm.io | bash -s stable && \
-    /bin/bash -l -c "rvm requirements && rvm install 2.2 && gem install -N fpm"
-
 COPY ./bootstrap.sh /tmp/bootstrap.sh

scripts/build/ci-build/Makefile

-VERSION="dev"
-TAG="grafana/build-container"
-USER_ID=$(shell id -u)
-GROUP_ID=$(shell id -g)
+VERSION = dev
+TAG = grafana/build-container
+USER_ID = $(shell id -u)
+GROUP_ID = $(shell id -g)
 
 all: build deploy
 
@@ -41,7 +41,7 @@ run-with-local-source-copy:
 	docker exec -ti grafana-build bash
 
 update-source:
-	docker cp "${GOPATH}/src/github.com/grafana/grafana" grafana-build:/go/src/github.com/grafana/	
+	docker cp "${GOPATH}/src/github.com/grafana/grafana" grafana-build:/go/src/github.com/grafana/
 
 attach:
 	docker exec -ti grafana-build bash

scripts/build/ci-build/README.md

 # grafana-build-container
-Grafana build container
 
-## Description
+These are the sources for the Docker image that we use for the Grafana build containers. The image source itself
+is in Dockerfile, but there are supporting scripts such as the Makefile, for building images.
 
-This is a container for cross-platform builds of Grafana. You can run it locally using the Makefile.
+The image is based on Debian Stretch, since we want an older Linux distribution (Stretch has long-term support into
+2022) to build binaries that are as portable as possible.
 
 ## Makefile targets