Fix staging containers failing on Windows Docker builds

Strip CRLF from entrypoint scripts in backend/frontend images, use Docker
DNS in staging nginx config, and wait for healthy app services before nginx.

Co-authored-by: Cursor <cursoragent@cursor.com>

.gitattributes

@@ -0,0 +1,2 @@
+# Shell scripts must use LF — CRLF breaks Alpine entrypoints ("No such file or directory").
+*.sh text eol=lf