FROM --platform=$TARGETPLATFORM in Podman and Docker

While building my erlang-cluster demonstration project for multi-architecture, using podman, I noticed some weirdness about using FROM --platform=$TARGETPLATFORM.

After various amounts of head-scratching, I discovered that – to get the correct architecture built – I needed to add --platform=$TARGETPLATFORM to the FROM command in my Dockerfile:

FROM --platform=$TARGETPLATFORM docker.io/erlang:27.3.3-alpine AS build

But when I later loaded this Dockerfile in VS Code, it complained that this was superfluous: Setting platform to predefined $TARGETPLATFORM in FROM is redundant as this is the default behavior.

It turns out that it’s a difference in behaviour between Podman and Docker.

As mentioned in part 2, podman uses whichever platform image you pulled last, whereas docker uses the platform image for the target platform (which defaults to the build platform).

I’ll demonstrate.

Recall that our simple Dockerfile looks like this:

FROM alpine:3.21

RUN uname -m

CMD ["uname", "-m"]

Here’s podman without the FROM --platform=...

# pull the wrong image first, to demonstrate.
$ podman pull --platform=linux/arm64 alpine:3.21
...

# build the image
$ podman build -t uname .
...

$ podman run --rm --name uname -it uname
WARNING: image platform (linux/arm64/v8) does not match the expected platform (linux/amd64)
aarch64

This is on amd64 (my Windows laptop running WSL2); you can see that it used whichever platform image was pulled last. But, as shown in the earlier post, Docker always uses the target platform (which defaults to the build platform) for the base image. This is the difference.

If I change the Dockerfile to this…

FROM --platform=$TARGETPLATFORM alpine:3.21

RUN uname -m

CMD ["uname", "-m"]

…then podman does the correct thing:

# pull the wrong image
$ podman pull --platform=linux/arm64 alpine:3.21
...

# build the image
$ podman build -t uname .
...

# it's correct this time.
$ podman run --rm --name uname -it uname
x86_64

Adding FROM --platform=$TARGETPLATFORM makes podman work like docker, but also explains why the VS Code extension for Docker complains that it’s superfluous – because, to Docker, it is:

$ docker build -t uname .
...

 => WARN: RedundantTargetPlatform: Setting platform to predefined $TARGETPLATFORM in FROM is redundant as this is the default behavior (line 2)
...

 1 warning found (use docker --debug to expand):
 - RedundantTargetPlatform: Setting platform to predefined $TARGETPLATFORM in FROM is redundant as this is the default behavior (line 2)

Conclusion

For podman, always specify --platform when building, either on the FROM directive, or with podman build --platform=...