Do not require lower-case DOMAIN variable - calculate it.

Dockerfile

@@ -11,7 +11,6 @@ ENV ADMIN_PASSWORD_SECRET=samba-admin-password \
     MODEL=standard \
     NETBIOS_NAME= \
     REALM=ad.example.com \
-    DOMAIN=ed.example.com \
     SERVER_STRING="Samba Member Server" \
     TZ=UTC \
     WINBIND_USE_DEFAULT_DOMAIN=yes \

entrypoint.sh

@@ -8,6 +8,7 @@ else
   NETBIOS_NAME=$(echo $NETBIOS_NAME | tr [a-z] [A-Z])
 fi
 REALM=$(echo "$REALM" | tr [a-z] [A-Z])
+DOMAIN=$(echo "$REALM" | tr [A-Z] [a-z])
 
 if [ ! -f /etc/timezone ] && [ ! -z "$TZ" ]; then
   echo 'Set timezone'
@@ -42,8 +43,6 @@ mkdir -p -m 700 /etc/samba/conf.d
 for file in /etc/samba/smb.conf; do
   sed -e "s:{{ ALLOW_DNS_UPDATES }}:$ALLOW_DNS_UPDATES:" \
       -e "s:{{ BIND_INTERFACES_ONLY }}:$BIND_INTERFACES_ONLY:" \
-      -e "s:{{ DOMAIN_LOGONS }}:$DOMAIN_LOGONS:" \
-      -e "s:{{ DOMAIN_MASTER }}:$DOMAIN_MASTER:" \
       -e "s+{{ INTERFACES }}+$INTERFACES+" \
       -e "s:{{ LOG_LEVEL }}:$LOG_LEVEL:" \
       -e "s:{{ NETBIOS_NAME }}:$NETBIOS_NAME:" \