docker-compose.yml: make it easy to restrict db resources

As we learned, postgres may behave differently depending
on the available RAM.
You can now test the database nearer to production by adding
DB_MEMORY_LIMIT=1G
DB_CPU_LIMIT=1
to your .env file.

IntelliJ does not like the env variable interpolation,
but it works.

I used 128G as fallback, because normally while developing you
don't want a slow DB.
With the cpus it was not possible to limit to more cpu than
the machine has. Thus, i limited to 8 cores which
every developer machine has hopefully.

docker-compose.yml

@@ -150,6 +150,11 @@ services:
       - target: 5432
         published: 5432
         protocol: tcp
+    deploy:
+      resources:
+        limits:
+          memory: ${DB_MEMORY_LIMIT:-128G}
+          cpus: ${DB_CPU_LIMIT:-8}
 
   mail:
     image: maildev/maildev