Update deprecated feature warning with steps to correct behavior

[kmanwar89]

Description

In docker compose, the external.name service configuration has been deprecated. If a compose file contains the legacy syntax, i.e.:

networks:
  default:
    external:
      name: <network name>

a warning message is seen advising the user the feature is deprecated, but no guidance is provided on what the new syntax should be. A user may attempt to simply delete the external line from the compose file, but this does not work for all services. An example of a service that breaks is authentik.

The original warning text is: WARN[0000] network default: network.external.name is deprecated in favor of network.name . I propose the warning message text be updated to advise the user to include external: true in the network stanza to avoid compatibility issues.

The proposed warning message might say: WARN[0000] network default: network.external.name is deprecated. If an external network needs to be defined, use "external: true" in the network stanza

[chr1shung]

Hi, I came across this issue today and I followed the warning message to update my network:

networks:
  external_network:
    external: true

It did eliminate the warning message but I noticed it also created the <project>_default network additionally.
I tried adding default back:

networks:
  default:
    external_network:
      external: true

But get the following error:

networks.default Additional property edgexpert_edgex-network is not allowed
docker: exit status 15

@ndeloof can you show me how to configure this correctly? Thanks

[ndeloof]

networks:
  default:
    name: external_network:
    external: true

see https://github.com/compose-spec/compose-spec/blob/master/06-networks.md