Fix edition error and image arg default value

[fschade]

Description

the edition validation does not mention the configured edition (go build flag) which results in:

{"level":"error","error":"unknown edition channel 'dev'","time":"2025-12-18T12:56:14.607885675Z","caller":"github.com/opencloud-eu/opencloud/pkg/version/version.go:55","message":"falling back to dev"}

Motivation and Context

if a configured edition is not allowed the error should mention the used edition, e.g.

{"level":"error","error":"unknown edition channel 'foo'","time":"2025-12-18T12:56:14.607885675Z","caller":"github.com/opencloud-eu/opencloud/pkg/version/version.go:55","message":"falling back to dev"}

also, the current EDITION ARG in Dockerfile.multiarch has no default value which always results in a logged error if the edition is not provided.

How Has This Been Tested?

• local docker builds
• unit-tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)