Enable Debian backport repo and install the far newer openldap components

Author: seanw2020

image/Dockerfile

@@ -11,12 +11,15 @@ ARG LDAP_OPENLDAP_UID
 RUN if [ -z "${LDAP_OPENLDAP_GID}" ]; then groupadd -r openldap; else groupadd -r -g ${LDAP_OPENLDAP_GID} openldap; fi \
     && if [ -z "${LDAP_OPENLDAP_UID}" ]; then useradd -r -g openldap openldap; else useradd -r -g openldap -u ${LDAP_OPENLDAP_UID} openldap; fi
 
-# Install OpenLDAP, ldap-utils and ssl-tools from baseimage and clean apt-get files
+# Add stretch-backports in preparation for downloading newer openldap components, especially sladp
+RUN echo "deb http://ftp.debian.org/debian stretch-backports main" >> /etc/apt/sources.list
+
+# Install OpenLDAP, ldap-utils and ssl-tools from the (backported) baseimage and clean apt-get files
 # sources: https://github.com/osixia/docker-light-baseimage/blob/stable/image/tool/add-service-available
 #          https://github.com/osixia/docker-light-baseimage/blob/stable/image/service-available/:ssl-tools/download.sh
 RUN echo "path-include /usr/share/doc/krb5*" >> /etc/dpkg/dpkg.cfg.d/docker && apt-get -y update \
     && /container/tool/add-service-available :ssl-tools \
-	  && LC_ALL=C DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+	  && LC_ALL=C DEBIAN_FRONTEND=noninteractive apt-get -t stretch-backports install -y --no-install-recommends \
        ldap-utils \
        libsasl2-modules \
        libsasl2-modules-db \