Environment fallback not working in .env file.

[Gerben]

Given system variable SYS_VAR=the_value when set:
and an .env file:
Some__env__var=${SYS_VAR:-fallback}

Using docker-compose v1.29.2, this results in either:
Some__env__var=the_value when SYS_VAR has a value or
Some__env__var=fallback when SYS_VAR is not set or is empty.

In docker compose v2.2.3 this results in
Some__env__var=the_value:-fallback when SYS_VAR has a value or
Some__env__var=:-fallback when SYS_VAR is not set or is empty.

To me, the behaviour in v2.2.3 is unexpected.

[Milas Bowman]

This is fixed in more recent versions of Compose v2.

We are also working on clarifying and formalizing the spec here around interpolation behavior, as the spec is currently inaccurate.