Accept a top-level network `name:` that is a single Compose variable
(`${VAR}`, `${VAR:-default}`, `${VAR-default}`), e.g. for external
networks that differ per server. The default must still be a valid
network name; surrounding text, nested variables, and shell syntax
are still rejected.
When parsing services, such variables are created as service
environment variables (seeded with the default) so users can view and
change them, while the compose file keeps the variable for Compose to
resolve from .env at deployment. Existing values are preserved on
re-parse.
Andras Bacsai
37bd776f70