Add current target platform as variable

[jhg03a]

Description

When compiling a multi-platform target today you can access the platform being compiled as an ARG inside a docker build via TARGETPLATFORM. I need the same information however higher up in the bake file to appropriately assign a cache per platform. Today I cannot do that and due to locking restrictions on debian's package cache as a cache mount, it means that it's not possible to parallelize the particular RUN stage accessing the cache. Additionally having such a magic variable would also allow for better context interpolation as well. If I have separate parent contexts per platform today, I have to mount both to each target and then use a RUN+TARGETPLATFORM switch to copy the right architecture files into place during a multiplatform build.

[tonistiigi]

Not quite sure what you are asking here. In bake you can access values of the target (or other targets) directly.

to appropriately assign a cache per platform

--cache-from/to works with multi-platform inputs. You don't need to separate it by arch if your build is multi-platform.