Accessing host machine as localhost from a Docker container that's also inside a boot2docker vm

Thanks to palimpestor's answer I figured it out:

Instead of --net="host", use --add-host="localhost:10.0.2.2"

Indeed:

• 10.0.2.2 is the default gateway defined for the guest network interface in NAT mode (reference).
  Read: it's your host, seen from boot2docker.
• --add-host... is adding localhost 10.0.2.2 in /etc/hosts (reference)

Note: you need to have set up a NAT adapter in your boot2docker VM VirtualBox settings (I did it through the GUI, don't know the CLI).

Instead of running with --net="host", try --add-host="localhost:192.168.59.3", which is the boot2docker host IP.