Add volumed-out config directory for symbolicator. (#566)

I would like to be able to customize the configuration for my Sentry 10 symbolicator instance, which this change allows me to easily do.

See related: https://github.com/getsentry/symbolicator/issues/245

Co-authored-by: Burak Yigit Kaya <ben@byk.im>

symbolicator/config.example.yml

@@ -0,0 +1,8 @@
+# See: https://getsentry.github.io/symbolicator/#configuration
+cache_dir: "/data"
+bind: "0.0.0.0:3021"
+logging:
+  level: "warn"
+metrics:
+  statsd: null
+sentry_dsn: null # TODO: Automatically fill this with the internal project DSN