Installation of admin impossible when user directory mounted

[markusritschel]

Hi Benjick,
I was using your Grav docker image now for a while being pretty satisfied by it.
However, I run into a problem when mounting a user directory from the host. The page is working fine, also the CLI in the running container works. But: when I try to install the admin plugin, I get an error. After installing Form and Login it aborts the installation when installing the Email package.

Preparing to install Email [v3.0.2]
  |- Downloading package...   100%
  |- Checking destination...  ok
  |- Installing package...  Installation aborted 

If I try then again, it ends with

Preparing to install Admin Panel [v1.9.7]
  |- Downloading package...   100%
  |- Checking destination...  ok
  |- Installing package... 
In Folder.php line 321:
  Unknown error  

Any idea why? Can you help me with this?