Allow public registration by default on docker env

My thought here is that a mailserver is started in
`docker-compose.override.yml` so you can see the emails being sent.

docker-compose.override.yml

@@ -15,6 +15,7 @@ services:
       - ./webapp:/nitro-web
     environment:
       - NUXT_BUILD=/tmp/nuxt # avoid file permission issues when `rm -rf .nuxt/`
+      - PUBLIC_REGISTRATION=true
     command: yarn run dev
   backend:
     build:
@@ -28,6 +29,7 @@ services:
       - SMTP_PORT=25
       - SMTP_IGNORE_TLS=true
       - "DEBUG=${DEBUG}"
+      - PUBLIC_REGISTRATION=true
   maintenance:
     image: humanconnection/maintenance:latest
     build: