Unsupported modifier error may not be correct

[tymokvo]

Related to #38 , an error is issued for variable expansion of the form ENV FOO=${FOO:?}

failed to process "${SERVICE_NAME:?}": unsupported modifier (?) in substitution

However, this does not seem to be true as building a Dockerfile with this syntax works (as I, at least) expect. E.g. building this file:

FROM ubuntu:20.04

ARG FOO

ENV FOO=${FOO:?}

...results in:

Step 3/3 : ENV FOO=${FOO:?}
failed to process "${FOO:?}": FOO: is not allowed to be unset

unless passed --build-arg FOO=bar.

Docker version 20.10.5, build 55c4c88

Let me know if I can provide any more info!

[rcjsuen]

@tymokvo I have reproduced the issue. Thank you for the bug report!

[rcjsuen]

@tymokvo The issue has now been fixed. Thanks again!

[tymokvo]

Wow, fast turnaround!