Fix; declare DOMAIN variable for docker setup (#10780)

In the /install form, the value for SSH Server Domain is taken form the DOMAIN variable
and overwrites SSH_DOMAIN environment variable set the first time if nothing done

Co-authored-by: Adrian POIGET <adrian.poiget@viveris.fr>

docker/root/etc/s6/gitea/setup

     # Substitude the environment variables in the template
     APP_NAME=${APP_NAME:-"Gitea: Git with a cup of tea"} \
     RUN_MODE=${RUN_MODE:-"dev"} \
+    DOMAIN=${DOMAIN:-"localhost"} \
     SSH_DOMAIN=${SSH_DOMAIN:-"localhost"} \
     HTTP_PORT=${HTTP_PORT:-"3000"} \
     ROOT_URL=${ROOT_URL:-""} \

docker/root/etc/templates/app.ini

 
 [server]
 APP_DATA_PATH = /data/gitea
+DOMAIN           = $DOMAIN
 SSH_DOMAIN       = $SSH_DOMAIN
 HTTP_PORT        = $HTTP_PORT
 ROOT_URL         = $ROOT_URL

docs/content/doc/installation/with-docker.en-us.md

 ---
-date: "2016-12-01T16:00:00+02:00"
+date: "2020-03-19T19:27:00+02:00"
 title: "Installation with Docker"
 slug: "install-with-docker"
 weight: 10
 
 * `APP_NAME`: **"Gitea: Git with a cup of tea"**: Application name, used in the page title.
 * `RUN_MODE`: **dev**: For performance and other purposes, change this to `prod` when deployed to a production environment.
-* `SSH_DOMAIN`: **localhost**: Domain name of this server, used for the displayed clone URL in Gitea's UI.
+* `DOMAIN`: **localhost**: Domain name of this server, used for the displayed http clone URL in Gitea's UI.
+* `SSH_DOMAIN`: **localhost**: Domain name of this server, used for the displayed ssh clone URL in Gitea's UI. If the install page is enabled, SSH Domain Server takes DOMAIN value in the form (which overwrite this setting on save).
 * `SSH_PORT`: **22**: SSH port displayed in clone URL.
 * `SSH_LISTEN_PORT`: **%(SSH\_PORT)s**: Port for the built-in SSH server.
 * `DISABLE_SSH`: **false**: Disable SSH feature when it's not available.