fix(ci): mark 'playwright' service under a 'testing' profile

the tests-cypress workflow was failing because there is no .env file
for the playwright service, but we don't even need that service to
start.

this makes it so it is not started by default.
see https://docs.docker.com/compose/how-tos/profiles/ for more.

compose.yml

@@ -52,3 +52,4 @@ services:
       - ./playwright:/playwright
     # https://code.visualstudio.com/docs/remote/create-dev-container#_use-docker-compose
     entrypoint: sleep infinity
+    profiles: ["testing"]