Expose alias domains as CLOUDRON_ALIAS_DOMAINS

This can be useful for app to set them in trusted hosts. Or alternately,
show different text when accessed from different domains.

src/docker.js

@@ -277,6 +277,8 @@ async function createSubcontainer(app, name, cmd, options) {
         `CLOUDRON_APP_DOMAIN=${domain}`
     ];
 
+    if (app.manifest.multiDomain) stdEnv.push(`CLOUDRON_ALIAS_DOMAINS=${app.aliasDomains.map(ad => ad.fqdn).join(',')}`);
+
     const secondaryDomainsEnv = app.secondaryDomains.map(sd => `${sd.environmentVariable}=${sd.fqdn}`);
 
     const portEnv = [];