Add environment variable support for Docker image (#2201)

* Add `gettext` dependencies as we need `envsubst` command;
* Modified s6's gitea setup script, instead of `cp` the template if no
`app.ini` exist, it will substitude the envvars and generate the new
`app.ini`;
* Make `/docker/etc/templates/app.ini` a template contains environment
variables;

Signed-off-by: Tao Wang <twang2218@gmail.com>

docker/etc/templates/app.ini

@@ -1,4 +1,6 @@
 [repository]
+APP_NAME = $APP_NAME
+APP_MODE = $APP_MODE
 ROOT = /data/git/repositories
 
 [repository.upload]
@@ -6,10 +8,19 @@ TEMP_PATH = /data/gitea/uploads
 
 [server]
 APP_DATA_PATH = /data/gitea
+SSH_DOMAIN       = $SSH_DOMAIN
+HTTP_PORT        = $HTTP_PORT
+ROOT_URL         = $ROOT_URL
+DISABLE_SSH      = $DISABLE_SSH
+SSH_PORT         = $SSH_PORT
 
 [database]
-DB_TYPE = sqlite3
 PATH = /data/gitea/gitea.db
+DB_TYPE = $DB_TYPE
+HOST    = $DB_HOST
+NAME    = $DB_NAME
+USER    = $DB_USER
+PASSWD  = $DB_PASSWD
 
 [session]
 PROVIDER_CONFIG = /data/gitea/sessions
@@ -22,3 +33,7 @@ PATH = /data/gitea/attachments
 
 [log]
 ROOT_PATH = /data/gitea/log
+
+[security]
+INSTALL_LOCK = $INSTALL_LOCK
+SECRET_KEY   = $SECRET_KEY