Shibboleth SP ignores MetadataProvider

I try to run a Shibboleth SP for the first time, but I immediately ran into an issue that I don't understand for three days now :/

I use the docker image unicon/shibboleth-sp as base to begin with. So far I've just modified shibboleth2.xml in two places. I wrote a specific IdP entityID into the <SSO> section and added a <MetadataProvider> that points to an external XML file containing the metadata of the IdP.

IMHO this should be enough to get redirected to the IdP when I try to access a protected URL on the SP. But instead, I get a Shibb-Exception No MetadataProvider available.

These are the changes I made to shibboleth2.xml:

<ApplicationDefaults entityID="https://sp.example.org/shibboleth" ... >
    ...
    <Sessions ... >
        ...
        <!--
        Configures SSO for a default IdP. To properly allow for >1 IdP, remove
        entityID property and adjust discoveryURL to point to discovery service.
        You can also override entityID on /Login query string, or in RequestMap/htaccess.
        -->
        <SSO entityID="https://testidp.aai.dfn.de/idp/shibboleth"
            discoveryProtocol="SAMLDS" discoveryURL="http://www.aai.dfn.de/DS/WAYF">
            SAML2
        </SSO>
        ...
    </Sessions>
    ...
    <MetadataProvider type="XML" id="dfn-aai-test-metadata"
        url="http://www.aai.dfn.de/fileadmin/metadata/dfn-aai-test-metadata.xml"
        backingFilePath="federation-dockermeta-metadata.xml" maxRefreshDelay="3600">
        <MetadataFilter type="RequireValidUntil" maxValidityInterval="2419200"/>
        <MetadataFilter type="Signature" certificate="/etc/ssl/aai/dfn-aai.g2.pem" verifyBackup="false"/>
    </MetadataProvider>
    ...
</ApplicationDefaults>

After debugging for days now I'm pretty sure that the SP correctly parses the <MetadataProvider> tag, but seems to completely ignore it. Setting Log-Levels to DEBUG show that the MetadataProvider gets parsed (its XML structure is visible in the log output), but it doesn't try to access the URL. There is not even a DNS request for www.aai.dfn.de nor does it try to access the URL. Also, there are no Errors in the log. There is not even a hint of it trying to load external Metadata in the logs. The first and only error I get in the logfiles is No MetadataProvider available after trying to access a protected resource.

I've never set up a Shibboleth SP before (because everybody told me it is a PITA). I'm not sure if it's a problem with the Shibboleth SP or the docker image. Most probably I'm the problem and I'm just missing something pretty obvious...

I need help :)

The complete code I used can be found here: https://gitlab.com/xsrf/shibb-sp/tree/5380f4550ac1a5ffb47d96138d837f1cf6acdb60