Ensure only one sftp rebuild is in progress

src/sftp.js

@@ -22,17 +22,30 @@ function startSftp(existingInfra, callback) {
     rebuild(callback);
 }
 
+var rebuildInProgress = false;
 function rebuild(callback) {
     assert.strictEqual(typeof options, 'object');
     assert.strictEqual(typeof callback, 'function');
 
+    if (rebuildInProgress) {
+        debug('waiting for other rebuild to finish');
+        return setTimeout(function () { rebuild(callback); }, 5000);
+    }
+
+    rebuildInProgress = true;
+
+    function done(error) {
+        rebuildInProgress = false;
+        callback(error);
+    }
+
     debug('rebuilding container');
 
     const tag = infra.images.sftp.tag;
     const memoryLimit = 256;
 
     apps.getAll(function (error, result) {
-        if (error) return callback(error);
+        if (error) return done(error);
 
         let dataDirs = [];
         result.forEach(function (app) {
@@ -73,6 +86,6 @@ function rebuild(callback) {
             shell.exec.bind(null, 'stopSftp', 'docker stop sftp || true'),
             shell.exec.bind(null, 'removeSftp', 'docker rm -f sftp || true'),
             shell.exec.bind(null, 'startSftp', cmd)
-        ], callback);
+        ], done);
     });
 }