Cause: A code change inadvertently removed variables set by the 'ENV' statement in a Dockerfile from the internal list of variables to use when later resolving variable arguments in the Dockerfile. Consequence: Variables defined in a Dockerfile like 'ENV VERSION=0.0.1' would not later resolve in statements like 'COPY file-${VERSION}.txt /'. In this second statement, the '${VERSION}' variable resolved to a blank string ("") rather than "0.0.1" as it should have. Fix: The code was updated to also add variables defined by the ENV statement into the internal list of variables. Result: 'ENV' statements now resolve as intended.