When I see this it's usually a permissions issue... i.e. the user that's running the shibd process doesn't have access to either the metadata file (or more likely in this case, the permissions file used to validate). I think just doing docker add in the Dockerfile isn't sufficient with that destination because shibd cannot read /etc/ssl/aai/.
I'd have to pull your repo and try it myself to work out what to do exactly, but it seems like a quick fudge would be to put the cert in /etc/shibboleth instead. shibd can read that dir.
EDIT: It looks like it can't download that metadata, in addition to the permission issue... I corrected the permissions problem by adding
RUN chown -R shibd:shibd /etc/shibboleth/
RUN chown -R shibd:shibd /var/cache/shibboleth/
to your Dockerfile.
Now I see this error when I try to test the config:
root@ac4861a1faae shibboleth]# /usr/sbin/shibd -t
2019-10-03 16:51:39 CRIT XMLTooling.Config : libcurl lacks OpenSSL-specific options, this will greatly limit functionality
2019-10-03 16:51:39 WARN Shibboleth.Application : insecure cookieProps setting, set to "https" for SSL/TLS-only usage
2019-10-03 16:51:39 WARN Shibboleth.Application : handlerSSL should be enabled for SSL/TLS-enabled web sites
2019-10-03 16:51:39 ERROR XMLTooling.libcurl.InputStream [dfn-aai-test-metadata]: error while fetching http://www.aai.dfn.de/fileadmin/metadata/dfn-aai-test-metadata.xml: (22) The requested URL returned error: 404 Not Found
2019-10-03 16:51:39 ERROR XMLTooling.ParserPool [dfn-aai-test-metadata]: fatal error on line 0, column 0, message: internal error in NetAccessor
2019-10-03 16:51:39 ERROR OpenSAML.MetadataProvider.XML [dfn-aai-test-metadata]: error while loading resource (http://www.aai.dfn.de/fileadmin/metadata/dfn-aai-test-metadata.xml): XML error(s) during parsing, check log for specifics
2019-10-03 16:51:39 WARN OpenSAML.MetadataProvider.XML [dfn-aai-test-metadata]: adjusted reload interval to 600 seconds
2019-10-03 16:51:39 WARN OpenSAML.MetadataProvider.XML [dfn-aai-test-metadata]: trying backup file, exception loading remote resource: XML error(s) during parsing, check log for specifics
2019-10-03 16:51:39 ERROR XMLTooling.ParserPool [dfn-aai-test-metadata]: fatal error on line 0, column 0, message: unable to open primary document entity '/var/cache/shibboleth/federation-dockermeta-metadata.xml'
2019-10-03 16:51:39 ERROR OpenSAML.MetadataProvider.XML [dfn-aai-test-metadata]: error while loading resource (/var/cache/shibboleth/federation-dockermeta-metadata.xml): XML error(s) during parsing, check log for specifics
2019-10-03 16:51:39 CRIT Shibboleth.Application : error initializing MetadataProvider: XML error(s) during parsing, check log for specifics
overall configuration is loadable, check console or log for non-fatal problems
And, sure enough... trying to CURL that URL from the docker container fails:
[root@ac4861a1faae shibboleth]# curl http://www.aai.dfn.de/fileadmin/metadata/dfn-aai-test-metadata.xml
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>404 Not Found</title>
</head><body>
<h1>Not Found</h1>
<p>The requested URL was not found on this server.</p>
</body></html>
Even though it succeeds from my local machine. Using the static file works, see: https://i.imgur.com/gXxC7z9.png which is basically what I'd expect to see for an SP that isn't actually integrated with that IdP.
So, the problem lies somewhere in your docker networking I think.
EDIT #2: Nope, just killed your dummy webserver that I somehow didn't connect was running and it worked just fine. It's a permissions thing like I said. Add those two RUN lines to the end of your Dockerfile and it should work.