new: ``!file-content`` modifier to allow including file contents in some config values

bin/compose-core

@@ -1742,6 +1742,13 @@ yaml_get_interpret() {
                 return 1
             }
             ;;
+        "!file-content")
+            source=$(echo "$content" | expand_vars) || {
+                err "shell expansion failed"
+                return 1
+            }
+            cat "$source" || return 1
+            ;;
         *)
             err "Invalid object tag ${WHITE}$tag${NORMAL}"
             return 1