Set default LANG in exec container to make umlauts and other special characters work

src/apps.js

@@ -2460,7 +2460,8 @@ async function createExec(app, options) {
         // When passing binary data, tty must be disabled. In addition, the stdout/stderr becomes a single
         // unified stream because of the nature of a tty (see https://github.com/docker/docker/issues/19696)
         Tty: options.tty,
-        Cmd: cmd
+        Cmd: cmd,
+        Env: [ 'LANG=C.UTF-8' ]
     };
 
     return await docker.createExec(app.containerId, createOptions);