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This is a tracking bug assuring the fix for [bug 1810069] gets applied in stream-container-tools-rhel8-rhel-8.3.0 branch of buildah.

Test with buildah-1.15.0-1.module+el8.3.0+7084+c16098dd.x86_64 and the env vars works as expect now. Details:
# buildah bud -t test .
STEP 1: FROM centos
STEP 2: ENV foo=oo
STEP 3: ADD ./f${foo} /tmp/foo
STEP 4: COMMIT test
Getting image source signatures
Copying blob eb29745b8228 skipped: already exists  
Copying blob 4aecdfe51b45 done  
Copying config 4e03c4472e done  
Writing manifest to image destination
Storing signatures
--> 4e03c4472e6
4e03c4472e680dff6e83efd1a3c91c992f2e2ed1722a2be7e3dad5e28b5d3190
# podman run -it --rm test /bin/bash -c "cat /tmp/foo"
bar

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Moderate: container-tools:rhel8 security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2020:4694