Repository 'Main Repository', ... is invalid [Docker]

Hi all!
I ran into an issue when installing programmes in openSUSE Leap, at least in the docker image. Whenever I want to install/update zypper, I get certain errors.

  • I start a docker shell
  • I run “zypper ref” (or update, install XYZ, …)
  • I get errors of type
    Repository 'Main Repository' is invalid.
    [repo-oss|http://download.opensuse.org/distribution/leap/15.5/repo/oss/] Valid metadata not found at specified URL   
    

What’s wrong here? How can I fix that?

Any help is appreciated! Cheers!


Complete steps to reproduce:

docker run -ti registry.opensuse.org/opensuse/leap:15.5.5.16 /bin/bash
Unable to find image 'registry.opensuse.org/opensuse/leap:15.5.5.16' locally
15.5.5.16: Pulling from opensuse/leap
850c013e9126: Already exists
Digest: sha256:e85c21012da262b0428e52e5c8c7c185087c1b69f4d602abe50f729a82d9a190
Status: Downloaded newer image for registry.opensuse.org/opensuse/leap:15.5.5.16
5e365d72a208:/ # zypper ref
Retrieving repository 'Update repository of openSUSE Backports' metadata ....................................................................................................................................................................[done]
Building repository 'Update repository of openSUSE Backports' cache .........................................................................................................................................................................[done]
Retrieving repository 'Non-OSS Repository' metadata .........................................................................................................................................................................................[done]
Building repository 'Non-OSS Repository' cache ..............................................................................................................................................................................................[done]
Warning: File 'repomd.xml' from repository 'Open H.264 Codec (openSUSE Leap)' is unsigned.

    Note: Signing data enables the recipient to verify that no modifications occurred after the data
    were signed. Accepting data with no, wrong or unknown signature can lead to a corrupted system
    and in extreme cases even to a system compromise.

    Note: File 'repomd.xml' is the repositories master index file. It ensures the integrity of the
    whole repo.

    Warning: We can't verify that no one meddled with this file, so it might not be trustworthy
    anymore! You should not continue unless you know it's safe.

File 'repomd.xml' from repository 'Open H.264 Codec (openSUSE Leap)' is unsigned, continue? [yes/no] (no): yes
Retrieving repository 'Open H.264 Codec (openSUSE Leap)' metadata ...........................................................................................................................................................................[done]
Building repository 'Open H.264 Codec (openSUSE Leap)' cache ................................................................................................................................................................................[done]
Retrieving repository 'Main Repository' metadata ...........................................................................................................................................................................................[error]
Repository 'Main Repository' is invalid.
[repo-oss|http://download.opensuse.org/distribution/leap/15.5/repo/oss/] Valid metadata not found at specified URL
History:
 - [|] Error trying to read from 'http://download.opensuse.org/distribution/leap/15.5/repo/oss/'
 - Download (curl) error for 'http://download.opensuse.org/distribution/leap/15.5/repo/oss/repodata/repomd.xml':
   Error code: Curl error 52
   Error message: Empty reply from server

Please check if the URIs defined for this repository are pointing to a valid repository.
Skipping repository 'Main Repository' because of the above error.
Retrieving repository 'Update repository with updates from SUSE Linux Enterprise 15' metadata ..............................................................................................................................................[error]
Repository 'Update repository with updates from SUSE Linux Enterprise 15' is invalid.
[repo-sle-update|http://download.opensuse.org/update/leap/15.5/sle/] Valid metadata not found at specified URL
History:
 - [|] Error trying to read from 'http://download.opensuse.org/update/leap/15.5/sle/'
 - Download (curl) error for 'http://download.opensuse.org/update/leap/15.5/sle/repodata/repomd.xml':
   Error code: Curl error 52
   Error message: Empty reply from server

Please check if the URIs defined for this repository are pointing to a valid repository.
Skipping repository 'Update repository with updates from SUSE Linux Enterprise 15' because of the above error.
Retrieving repository 'Main Update Repository' metadata .....................................................................................................................................................................................[done]
Building repository 'Main Update Repository' cache ..........................................................................................................................................................................................[done]
Retrieving repository 'Update Repository (Non-Oss)' metadata ................................................................................................................................................................................[done]
Building repository 'Update Repository (Non-Oss)' cache .....................................................................................................................................................................................[done]
Some of the repositories have not been refreshed because of an error.
5e365d72a208:/ #