fix: update database host configuration to use a consistent format

templates/app.ini.tpl

@@ -17,7 +17,7 @@ DB_TYPE = ${GITEA_DB_TYPE}
 PATH    = /data/gitea/gitea.db
 ; SQLITE_BLOCK_END
 ; EXTDB_BLOCK_START
-HOST    = ${GITEA_DB_HOST}:${GITEA_DB_PORT}
+HOST    = db:${GITEA_DB_PORT}
 NAME    = ${GITEA_DB_NAME}
 USER    = ${GITEA_DB_USER}
 PASSWD  = ${GITEA_DB_PASSWD}