Here's something I chased around for an hour or so.
You get the minimal version:
% cat Dockerfile
ARG THANG=zero
FROM alpine
ARG THING=one
RUN export THING=two && echo ${THING?} && echo ${THANG?}
% docker build --no-cache --progress=plain .
Account for the weirdnesses in the output. Here's a non-spoilery except:
------
> [2/2] RUN export THING=two && echo one && echo ${THANG?}:
0.072 two
0.072 /bin/sh: THANG: parameter not set
------
