Always npm install in docker runner

Author: oliversalzburg

lib/install-packages.js

@@ -5,11 +5,17 @@ var packageHash = require('./package-hash');
 var npmCommand = require('./npm-command');
 
 function installPackages(config, context, done) {
+  var dataDir = context.dataDir;
+  var projectDir = path.join(dataDir, config.subdir || '');
+
+  // We don't support any caching in a docker container, as the container is reconstructed on every run anyway.
+  if (context.runnerId === 'docker') {
+    return runNpm('install', context, projectDir, done);
+  }
+
   var update = config.update_cache;
   var cachier = context.cachier('modules');
-  var dataDir = context.dataDir;
   var policy = config.caching;
-  var projectDir = path.join(dataDir, config.subdir || '');
 
   if (policy !== 'strict' && policy !== 'loose') {
     return runNpm('install', context, projectDir, done);