add OpenTelemetry toggle to Docker workflow

[fabracht]

Summary

• Adds opentelemetry boolean input to the Docker workflow_dispatch trigger
• Passes --features opentelemetry as a build-arg to the Dockerfile when enabled
• Tag-push releases are unaffected (no features flag)

Test plan

• Trigger workflow manually with opentelemetry: true, tag: dev-otel — verify --features opentelemetry in cargo build logs
• Trigger without opentelemetry — verify no features flag appears
• Tag push path unchanged