Use a different architecture naming scheme in ARCH variable

Debian and Ubuntu images no longer support the architecture naming scheme previously used as a value for --arch option during image pulling from Docker registry so use the supported scheme while using the previous scheme for ARCH_NAMESPACE to prevent changing links of Salsa CI ARM images.

This MR also contains a commit to use arm32v5 namespace for bullseye arm/v5 image. Because this image doesn't currently exist on Docker registry's "library" namespace. Revert this commit when the cause of it's existence is no longer relevant.

closes: #391 (closed)