chore: import.sh: make psql always use $SHELL when evaluating shell code

Dockerfile

@@ -55,5 +55,9 @@ RUN npm install --omit dev && npm cache clean --force
 
 ADD . .
 
+# When evaluating SQL scripts in postprocessing.d, import.sh passes $SHELL into psql explicitly, which in turn executes backtick-ed code blocks using $SHELL.
+# Because the script inlined within those backticks/backquotes might rely on certain behavior, to achieve stability, we define this explicitly here, rather than relying on the implicit default from our base image.
+ENV SHELL=/bin/bash
+
 ENTRYPOINT []
 CMD ["/usr/local/bin/node", "importer.js"]

import.sh

@@ -130,7 +130,7 @@ if [ -d "$postprocessing_d_path" ]; then
 	for file in "$postprocessing_d_path/"*; do
 		ext="${file##*.}"
 		if [ "$ext" = "sql" ]; then
-			psql -b -1 -v 'ON_ERROR_STOP=1' "${psql_args[@]}" \
+			psql -b -1 -v 'ON_ERROR_STOP=1' --set=SHELL="$SHELL" "${psql_args[@]}" \
 				-f "$file"
 		else
 			"$file" "$gtfs_path"