dockerfile: show target platform on cross compiling

[tonistiigi]

On cross-compilation this PR now shows both build and target platform on progressbar, not only the platform or the current stage.

When both platforms have the same OS it is only shown once for less verbosity.

 => [linux/arm64 2/3] RUN apk add vim                                                                                                                         2.7s
 => [linux/arm64 3/3] RUN sleep 1                                                                                                                             1.3s
 => [linux/arm64->amd64 3/3] RUN sleep 1                                                                                                                      1.3s
 => [linux/arm64->windows/amd64 3/3] RUN sleep 1                                                                                                              1.3s

Signed-off-by: Tonis Tiigi [email protected]

[crazy-max]

Neat! I had exactly this issue to recognize which target failed during the cross compilation.

[AkihiroSuda]

Why ignore this err?

[tonistiigi]

It's a convention. A Dockerfile might be just using ARG TARGETPLATFORM with a completely custom value definition. We shouldn't fail the build in this case if we don't understand this value.

I can add a comment for that if you like.