Drop use of deprecated network.external.name

[kdambekalns]

The config claims to be version 3.7, so this drops a config option that
has been deprecated with 3.5.

Fixes #33
See https://docs.docker.com/compose/compose-file/compose-file-v3/#external-1

[robertlemke]

The property "name" was not dropped completely:

So, should be:

networks:
  local_beach:
    external: true
    name: local_beach

[kdambekalns]

But the name is taken from the key… as this example in the documentation shows:

networks:
  outside:
    external: true

The name can be used to

specify the name of the network separately from the name used to refer to it within the Compose file

But we don't need that…