env/crosscompile: update the three cross compile envs from Jessie to Buster

The earlier fix to remove IN_KUBERNETES in buildlet/stage0/stage0.go
in CL 201739 caused Kubernetes-based cross compilation builds on fail
to start up because I forgot to rebuild & push those container images.

But when I went to re-push them, I found they're too old (Debian
Jessie) and don't build anymore. So update them to Debian Buster.

While I'm here, remove the Debian release name from their image name,
as this won't be the last time we need to upgrade them. Unlike real
builders, the OS release isn't the point of these containers, as they
don't run tests. They exist only to do builds, so the OS isn't
interesting or worth calling out.

Updates golang/go#34956

Change-Id: Iee4691a821421b161a9333b4441f83ff45d1ffec
Reviewed-on: https://go-review.googlesource.com/c/build/+/202021
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index fa40c6b..e643fe6 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -135,14 +135,14 @@
 		SSHUsername:     "root",
 	},
 	"host-linux-armhf-cross": &HostConfig{
-		Notes:           "Debian Jessie with armhf cross-compiler, built from env/crosscompile/linux-armhf-jessie",
-		ContainerImage:  "linux-armhf-jessie:latest",
+		Notes:           "Debian with armhf cross-compiler, built from env/crosscompile/linux-armhf",
+		ContainerImage:  "linux-armhf-cross:latest",
 		buildletURLTmpl: "http://storage.googleapis.com/$BUCKET/buildlet.linux-amd64",
 		env:             []string{"GOROOT_BOOTSTRAP=/go1.4"},
 	},
 	"host-linux-armel-cross": &HostConfig{
-		Notes:           "Debian Jessie with armel cross-compiler, from env/crosscompile/linux-armel-stretch",
-		ContainerImage:  "linux-armel-stretch:latest",
+		Notes:           "Debian with armel cross-compiler, from env/crosscompile/linux-armel",
+		ContainerImage:  "linux-armel-cross:latest",
 		buildletURLTmpl: "http://storage.googleapis.com/$BUCKET/buildlet.linux-amd64",
 		env:             []string{"GOROOT_BOOTSTRAP=/go1.4"},
 	},
@@ -166,7 +166,7 @@
 	},
 	"host-s390x-cross-kube": &HostConfig{
 		Notes:           "Container with s390x cross-compiler.",
-		ContainerImage:  "linux-s390x-stretch:latest",
+		ContainerImage:  "linux-s390x-cross:latest",
 		buildletURLTmpl: "https://storage.googleapis.com/$BUCKET/buildlet.linux-amd64",
 		env:             []string{"GOROOT_BOOTSTRAP=/go1.4"},
 	},
diff --git a/env/crosscompile/linux-armel-stretch/Dockerfile b/env/crosscompile/linux-armel-cross/Dockerfile
similarity index 90%
rename from env/crosscompile/linux-armel-stretch/Dockerfile
rename to env/crosscompile/linux-armel-cross/Dockerfile
index 4cdbee9..57e8ec4 100644
--- a/env/crosscompile/linux-armel-stretch/Dockerfile
+++ b/env/crosscompile/linux-armel-cross/Dockerfile
@@ -2,12 +2,12 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-# Linux builder VM running Debian stretch (i.e. Debian testing)
+# Debian buildlet image with cross compilers for linux-arm-arm5.
 # Docker tag gobuilders/linux-armel-stretch
 
 FROM golang/buildlet-stage0 AS stage0
 
-FROM debian:stretch
+FROM debian:buster
 MAINTAINER golang-dev <golang-dev@googlegroups.com>
 
 ENV DEBIAN_FRONTEND noninteractive
@@ -15,7 +15,7 @@
 # curl: for getting and unpacking Go 1.4 source
 # git-core: for interacting with the Go source & subrepos
 # gcc, libc-dev: for building Go's bootstrap 'dist' prog
-# gcc-armel-linux-gnu, libc6-dev-armel-cross: for armel builds
+# gcc-arm-linux-gnueabi, libc6-dev-armel-cross: for armel builds
 # linux-libc-dev(armel): for asm/errno.h
 # procps, lsof, psmisc: misc tools
 RUN dpkg --add-architecture armel \
diff --git a/env/crosscompile/linux-armel-stretch/Makefile b/env/crosscompile/linux-armel-cross/Makefile
similarity index 100%
rename from env/crosscompile/linux-armel-stretch/Makefile
rename to env/crosscompile/linux-armel-cross/Makefile
diff --git a/env/crosscompile/linux-armel-stretch/README b/env/crosscompile/linux-armel-cross/README
similarity index 100%
rename from env/crosscompile/linux-armel-stretch/README
rename to env/crosscompile/linux-armel-cross/README
diff --git a/env/crosscompile/linux-armhf-jessie/Dockerfile b/env/crosscompile/linux-armhf-cross/Dockerfile
similarity index 78%
rename from env/crosscompile/linux-armhf-jessie/Dockerfile
rename to env/crosscompile/linux-armhf-cross/Dockerfile
index 2aed9ce..05355a9 100644
--- a/env/crosscompile/linux-armhf-jessie/Dockerfile
+++ b/env/crosscompile/linux-armhf-cross/Dockerfile
@@ -2,21 +2,16 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-# Linux builder VM running Debian stretch (i.e. Debian testing)
-# Docker tag gobuilders/linux-armhf-stretch
+# Linux builder VM running Debian, with cross compilers for linux-arm.
+# Docker tag gobuilders/linux-armhf-cross
 
 FROM golang/buildlet-stage0 AS stage0
 
-FROM debian:jessie
+FROM debian:buster
 MAINTAINER golang-dev <golang-dev@googlegroups.com>
 
 ENV DEBIAN_FRONTEND noninteractive
 
-# Add the Debian cross toolchains repository and key
-COPY emdebian-toolchain-archive.key /tmp/
-RUN echo "deb http://emdebian.org/tools/debian/ jessie main" >> /etc/apt/sources.list \
-	&& apt-key add /tmp/emdebian-toolchain-archive.key
-
 # git-core: for interacting with the Go source & subrepos
 # gcc, libc-dev: for building Go's bootstrap 'dist' prog
 # gcc-armhf-linux-gnu: for armhf builds
diff --git a/env/crosscompile/linux-armhf-jessie/Makefile b/env/crosscompile/linux-armhf-cross/Makefile
similarity index 100%
rename from env/crosscompile/linux-armhf-jessie/Makefile
rename to env/crosscompile/linux-armhf-cross/Makefile
diff --git a/env/crosscompile/linux-armhf-jessie/README b/env/crosscompile/linux-armhf-cross/README
similarity index 100%
rename from env/crosscompile/linux-armhf-jessie/README
rename to env/crosscompile/linux-armhf-cross/README
diff --git a/env/crosscompile/linux-armhf-jessie/emdebian-toolchain-archive.key b/env/crosscompile/linux-armhf-jessie/emdebian-toolchain-archive.key
deleted file mode 100644
index a4d3112..0000000
--- a/env/crosscompile/linux-armhf-jessie/emdebian-toolchain-archive.key
+++ /dev/null
@@ -1,41 +0,0 @@
------BEGIN PGP PUBLIC KEY BLOCK-----
-Version: GnuPG v1.4.12 (GNU/Linux)
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------END PGP PUBLIC KEY BLOCK-----
diff --git a/env/crosscompile/linux-s390x-stretch/Dockerfile b/env/crosscompile/linux-s390x-cross/Dockerfile
similarity index 91%
rename from env/crosscompile/linux-s390x-stretch/Dockerfile
rename to env/crosscompile/linux-s390x-cross/Dockerfile
index ea463ef..24a9f5b 100644
--- a/env/crosscompile/linux-s390x-stretch/Dockerfile
+++ b/env/crosscompile/linux-s390x-cross/Dockerfile
@@ -2,12 +2,12 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-# Linux builder VM running Debian stretch (i.e. Debian testing)
-# Docker tag gobuilders/linux-s390x-stretch
+# Linux buildlet image with with cross compilers for linux-s390x.
+# Docker tag gobuilders/linux-s390x-cross
 
 FROM golang/buildlet-stage0 AS stage0
 
-FROM debian:stretch
+FROM debian:buster
 MAINTAINER golang-dev <golang-dev@googlegroups.com>
 
 ENV DEBIAN_FRONTEND noninteractive
diff --git a/env/crosscompile/linux-s390x-stretch/Makefile b/env/crosscompile/linux-s390x-cross/Makefile
similarity index 100%
rename from env/crosscompile/linux-s390x-stretch/Makefile
rename to env/crosscompile/linux-s390x-cross/Makefile
diff --git a/env/crosscompile/linux-s390x-stretch/README b/env/crosscompile/linux-s390x-cross/README
similarity index 100%
rename from env/crosscompile/linux-s390x-stretch/README
rename to env/crosscompile/linux-s390x-cross/README