Error starting datanode on hadoop - hadoop

Question

I'm trying to run a hadoop cluster via Docker. I have one virtual machine as the namenode and another for the datanode, but the datanode gives me this error running start-dfs.sh:
namenode: namenode running as process 130. Stop it first.
The command jps on the datanode does not show the namenode running. Then I try to start it by hand, using:
hadoop namenode
And it fails with this error:
java.net.BindException: Problem binding to [namenode:9000] java.net.BindException: Cannot assign requested address; For more details see: http://wiki.apache.org/hadoop/BindException
So far it seems that namenode is not accesible or is not listening on port 9000. But the network setup is correct: if I execute on datanode:
telnet namenode 9000
It correctly connects to the namenode, and the command netstat -apn | grep 9000 from namenode shows the incoming connection. If I shut down dfs on namenode (stop-dfs.sh), the telnet command from datanode fails with "Connection closed by foreign host."
hdfs-site.xml
<configuration>
<property>
<name>dfs.replication</name>
<value>3</value> <!-- I have tried with 1 and 2 too -->
</property>
<property>
<name>dfs.namenode.datanode.registration.ip-hostname-check</name>
<value>false</value>
</property>
</configuration>
core-site.xml
<configuration>
<property>
<name>fs.defaultFS</name>
<value>hdfs://namenode:9000</value>
</property>
</configuration>
Thanks!