title: docker, mongorestore - ERROR: root directory must be a dump of a single database
body: <p>i had to update a collection on my local machine and want to upload this to a docker instance, here is my procedure</p>
  
  <pre><code>$ #backup local database
  $ mongodump --collection styles --db ccc-mongo
  $ tar -zcvf dump.tar.gz dump
  
  $ scp dump.tar.gz me@remote:/home/me
  $ ssh me@remote
  me@remote: ~ $ docker exec -i mongo-dev sh -c "cd /tmp &amp;&amp; tar xzf - &amp;&amp; mongorestore --collection styles --db ccc-mongo" &lt; dump.tar.gz
  </code></pre>
  
  <p>this gives me an error</p>
  
  <pre><code>connected to: 127.0.0.1
  Fri Dec 18 16:05:26.768 ERROR: ERROR: root directory must be a dump of a single database
  Fri Dec 18 16:05:26.768 ERROR:        when specifying a db name with --db
  Import BSON files into MongoDB.
  </code></pre>
  
  <p>i just need to restore one collection without removing the other data, any advice much appreciated</p>