doc: Enhance Docker platform specification in documentation

[miry]

Description

Allow to run ghcr.io/synocommunity/spksrc from ARM machines:

$ docker run -it -v $(pwd):/spksrc -w /spksrc -e TAR_CMD="fakeroot tar" ghcr.io/synocommunity/spksrc /bin/bash

Unable to find image 'ghcr.io/synocommunity/spksrc:latest' locally
latest: Pulling from synocommunity/spksrc
docker: no matching manifest for linux/arm64/v8 in the manifest list entries.
See 'docker run --help'.

Workaround without this fix is to specify arch with --platform=linux/amd64:

$ docker run --platform=linux/amd64  -it -v $(pwd):/spksrc -w /spksrc -e TAR_CMD="fakeroot tar" ghcr.io/synocommunity/spksrc /bin/bash

P.S.: I tried to add platform, but there is some broken package installation for i386.

Checklist

• Build rule all-supported completed successfully
• New installation of package completed successfully
• Package upgrade completed successfully (Manually install the package again)
• Package functionality was tested
• Any needed documentation is updated/created

Type of change

• Bug fix
• New Package
• Package update
• Includes small framework changes
• This change requires a documentation update (e.g. Wiki)

[hgy59]

@miry what is your intention with this PR?

spksrc is a framework to build packages for Synology DSM by using the toolchains provided by Synology.

It's about cross compilation of source code with the tools in the the toolchains for dedicated CPU architectures.
I.e. to build executables for ARM32, ARM64, PPC32, i386 (x86) and X64 (x86_64) targets.
But all those tools, like cross-compiler, cross-linker etc, are executables built for x86_64 CPUs, independent of the target architecture.

It will never work on platforms without x86_64 CPU.

[miry]

@hgy59 Probably you saw my draft pr, so I removed my first version to support ARM docker image and keep only changes in documentation.
It helps to run docker image on arm machines and contribute to the project.
It should still allow to run and build x86_64 from inside docker from arm machines, because docker uses virtual machine in background for different platforms.