new: [apache] target port could end up empty if service image was not yet pulled

We decide to pull the image here, as we are garanteed that if it had to
be built, it was built.

apache/lib/common

@@ -465,6 +465,9 @@ __vhost_proxy_statement() {
     if [ -z "$TARGET" ]; then
         ## First exposed port:
         base_image=$(service_base_docker_image "$BASE_SERVICE_NAME") || return 1
+        if ! docker_has_image "$base_image"; then
+            docker pull "$base_image"
+        fi
         first_exposed_port=$(image_exposed_ports_0 "$base_image" | tr '\0' '\n' | head -n 1 | cut -f 1 -d /) || return 1
         TARGET=$MASTER_BASE_SERVICE_NAME:$first_exposed_port
         info "No target was specified, introspection found: $TARGET"