$ cat test.Dockerfile 
ARG REL=buster
FROM debian:$REL
ARG REL=buster
RUN echo PREVED > /preved; echo PREVED $REL
RUN sleep 300
$ podman build -f test.Dockerfile .
STEP 1: FROM debian:buster
STEP 2: ARG REL=buster
--> Using cache 90a1a0e12affbd18eda1364116b5d00edc0855cf38d8045fd377528cf0799cfb
--> 90a1a0e12af
STEP 3: RUN echo PREVED > /preved; echo PREVED $REL
PREVED buster
--> Using cache e5989542ab35a32bc63f964b5a48a790afba54a78a16c7195514c8e0347a0beb
--> e5989542ab3
STEP 4: RUN sleep 300
^C
$ podman build -f test.Dockerfile .
STEP 1: FROM debian:buster
STEP 2: ARG REL=buster
--> Using cache 90a1a0e12affbd18eda1364116b5d00edc0855cf38d8045fd377528cf0799cfb
--> 90a1a0e12af
STEP 3: RUN echo PREVED > /preved; echo PREVED $REL
PREVED buster
--> Using cache e5989542ab35a32bc63f964b5a48a790afba54a78a16c7195514c8e0347a0beb
--> e5989542ab3
STEP 4: RUN sleep 300
^C

$ buildah bud --layers -f test.Dockerfile .
STEP 1: FROM debian:buster
Resolved "debian" as an alias (/etc/containers/registries.conf.d/shortnames.conf)
Getting image source signatures
Copying blob 0ecb575e629c done  
Copying config 5890f8ba95 done  
Writing manifest to image destination
Storing signatures
STEP 2: ARG REL=buster
--> b8a95a616b2
STEP 3: RUN echo PREVED > /preved; echo PREVED $REL
PREVED buster
--> 4523ed4df15
STEP 4: RUN sleep 300
^C
$ buildah bud --layers -f test.Dockerfile .
STEP 1: FROM debian:buster
STEP 2: ARG REL=buster
--> Using cache b8a95a616b291441e9badd3a8a01c10b1d3dc6f3e8fc84dcb3469f8c37bf9490
--> b8a95a616b2
STEP 3: RUN echo PREVED > /preved; echo PREVED $REL
PREVED buster
--> Using cache 4523ed4df15d14ef89b8a7df0987337241a1686608ef0ced8e3d1c988100155f
--> 4523ed4df15
STEP 4: RUN sleep 300
^C