migration: prefer ./external for RTI/ArcGIS; detect RTI arch in FindRTI; install/unpack ArcGIS into ./external

app/run_docker.sh

@@ -225,8 +225,8 @@ docker run --rm $DOCKER_TTY_FLAG \
   -e WORKDIR_IN_CONTAINER="$WORKDIR_IN_CONTAINER" \
   -e QT_ROOT="$QT_ROOT_OVERRIDE" \
   -e Qt6_ROOT="$QT_ROOT_OVERRIDE" \
-  -e CMAKE_PREFIX_PATH="$QT_ROOT_OVERRIDE" \
-  -e PROJECT_BASE_DIR="${PROJECT_BASE_DIR:-./}" \
+  -e EXTERNAL_DIR="/repo/external" \
+  -e CMAKE_PREFIX_PATH="$QT_ROOT_OVERRIDE:/repo/external" \
   -e PROJECT_BASE_DIR="${PROJECT_BASE_DIR:-./}" \
   "$IMAGE_NAME" \
   bash -s -- "${CMD_ARGS[@]}" <<'DOCKER_SCRIPT'