using deprecated docker-compose syntax

[tobiasr-siemens]

Developer-Guide-Hands-on-App/src/solution/HandsOn_1/my_edge_app/docker-compose.yml

Line 60 in 6a1a625

external:

This syntax for an external network is deprecated as of V2.1. Reference:
https://github.com/docker/compose/blob/v1/docs/Compose file reference (legacy)/version-2.md#external

Trying to install an app that uses it to an IED presumably is the cause for this error message: