Add security mirror override for debian-minimal

Add option to set the security mirror URL independently in the
debian-minimal element, since this can not be overriden by the
standard DIB_DISTRIBUTION_MIRROR variable.

Change-Id: I145844a410d06a479e68db1bf6d5d0159389305c

diskimage_builder/elements/debian-minimal/README.rst

@@ -12,4 +12,11 @@ be created, the variable ``DIB_RELEASE`` can be set appropriately.
 
 Note that this element installs ``systemd-sysv`` as the init system
 
+The element obeys the ``DIB_DISTRIBUTION_MIRROR`` argument for
+mirroring (see ``debootsrap`` element documentation).  However, the
+security repositories are separate for Debian, so we can not assume
+they exist at ``DIB_DISTRIBUTION_MIRROR``.  If you do not wish to use
+the upstream repository (from ``security.debian.org``) override it
+with ``DIB_DEBIAN_SECURITY_MIRROR``.
+
 .. element_deps::

diskimage_builder/elements/debian-minimal/environment.d/10-debian-minimal.bash

@@ -6,6 +6,12 @@ if [ -n "${DIB_DEBIAN_DISTRIBUTION_MIRROR:-}" ]; then
 fi
 export DIB_DISTRIBUTION_MIRROR=${DIB_DISTRIBUTION_MIRROR:-http://deb.debian.org/debian}
 
+# With Debian, security is in a different repository.  We can't, say,
+# assume "${DIB_DISTRIBUTION_MIRROR}-security" is valid.  The only
+# choice is for people to add it separately, otherwise we use
+# upstream.
+DIB_DEBIAN_SECURITY_MIRROR=${DIB_DEBIAN_SECURITY_MIRROR:-http://security.debian.org/}
+
 export DIB_DEBIAN_COMPONENTS=${DIB_DEBIAN_COMPONENTS:-main}
 export DIB_DEBIAN_COMPONENTS_WS=${DIB_DEBIAN_COMPONENTS//,/ }
 
@@ -13,7 +19,7 @@ DIB_APT_SOURCES_CONF_DEFAULT=\
 "default:deb ${DIB_DISTRIBUTION_MIRROR} ${DIB_RELEASE} ${DIB_DEBIAN_COMPONENTS_WS}
 backports:deb ${DIB_DISTRIBUTION_MIRROR} ${DIB_RELEASE}-backports ${DIB_DEBIAN_COMPONENTS_WS}
 updates:deb ${DIB_DISTRIBUTION_MIRROR} ${DIB_RELEASE}-updates ${DIB_DEBIAN_COMPONENTS_WS}
-security:deb http://security.debian.org/ ${DIB_RELEASE}/updates ${DIB_DEBIAN_COMPONENTS_WS}
+security:deb ${DIB_DEBIAN_SECURITY_MIRROR} ${DIB_RELEASE}/updates ${DIB_DEBIAN_COMPONENTS_WS}
 "
 
 if [ "${DIB_RELEASE}" = "testing" -o "${DIB_RELEASE}" = "unstable" ]; then

releasenotes/notes/debian-security-7279855cf464f88b.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    A new variable ``DIB_DEBIAN_SECURITY_MIRROR`` is added to override
+    the path to the security repositories.  Previously this was
+    hard-coded to the upstream repositories, which could get out of
+    sync with mirrors specified with ``DIB_DISTRIBUTION_MIRROR``.