ci: fix platform of the target docker image

[galargh]

@lidel As per https://www.docker.com/blog/faster-multi-platform-builds-dockerfile-cross-compilation-guide/, shouldn't the final image build skip the --platform arg so that the end image is built for the requested target platform?

closes #7541
closes #4584

[lidel]

I wonder if this helps with #9901.

Don't have ARM at hand to test, would have to dust-off a raspberry Pi or setup QEMU to confirm – parking until me or someone else has time to confirm.

[galargh]

Oh nice, I'll have it tested tomorrow and get back to you.

[lidel]

@galargh I've tested this approach (removal of --platform from second FROM) in simpler but similar Dockerfile from bifrost-gateway, and it was not enough.

We have some legacy overengineering for building tini and su-exec (we literally build su-exec from C code and that is unable to leverage golang tooling for cross-compilation).

Good news

I was successful in cleaning up that in ipfs/bifrost-gateway#159, and did a smoke test on old Rpi4 (ARM64) and it runs fine, while keeping the similar size.

Bad news

Kubo's Dockerfiles has more moving pieces.

I think we could try to do the same thing here, but:

• I don't know if we can set CGO_ENABLED=1 and remove dependency on glibc
  • maybe @Jorropo or @aschmahmann know if this is feasible? will this break plugins?
  • if we need to keep glibc, perhaps we can install necessary lib, eg.g RUN apk add --no-cache gcompat ?
• we seem to install and copy fuse, libdl, libssh and libcrypto from first stage to second
  • I suspect we could remove it from first stage, and install arch-specific libraries in second one