docker: debian-bootstrap.pre allow customising of variant/url

We default to the buildd variant as most of our images are for
building. However lets give the user the ability to specify "minbase"
if they want to create a simple base image for experimentation.

Allowing the tweaking of DEB_URL means we can also bootstrap other
Debian based OS's. For example:

  make docker-binfmt-image-debian-ubuntu-bionic-arm64 \
       DEB_ARCH=arm64 DEB_TYPE=bionic \
       DEB_VARIANT=minbase DEB_URL=http://ports.ubuntu.com/ \
       EXECUTABLE=./aarch64-linux-user/qemu-aarch64
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>

tests/docker/dockerfiles/debian-bootstrap.pre

@@ -32,6 +32,15 @@ if [ -z "${DEB_TYPE}" ]; then
 
 fi
 
+# The following allow finer grain control over the defaults
+if [ -z "${DEB_VARIANT}" ]; then
+    DEB_VARIANT=buildd
+fi
+
+if [ -z "${DEB_URL}" ]; then
+    DEB_URL="http://httpredir.debian.org/debian"
+fi
+
 # We check in order for
 #
 #  - DEBOOTSTRAP_DIR pointing at a development checkout
@@ -107,5 +116,5 @@ fi
 
 echo "Building a rootfs using ${FAKEROOT} and ${DEBOOTSTRAP} ${DEB_ARCH}/${DEB_TYPE}"
 
-${FAKEROOT} ${DEBOOTSTRAP} --variant=buildd --foreign --arch=$DEB_ARCH $DEB_TYPE . http://httpredir.debian.org/debian || exit 1
+${FAKEROOT} ${DEBOOTSTRAP} --variant=$DEB_VARIANT --foreign --arch=$DEB_ARCH $DEB_TYPE . $DEB_URL || exit 1
 exit 0