Allow overriding daemon docker registry via env

[rksharma-owg]

Problem

arctl daemon start always initializes the compose manager with the default docker registry (localhost:5001) via dockercompose.DefaultConfig() so CI and non-local environments cannot point daemon image pulls to another registry without code changes.

Root cause

In the root command wiring, the daemon manager was created with a fixed default config, and there was no environment-configurable override for DockerRegistry in this start path.

Focused solution

Add a small helper in pkg/cli/root.go that reads ARCTL_DAEMON_DOCKER_REGISTRY from environment (trimmed) and applies it to the daemon compose config before constructing clidaemon.NewCommand.

The default behavior remains unchanged (localhost:5001) when the variable is unset.

Files changed

• pkg/cli/root.go
• pkg/cli/root_test.go

Tests added

• TestDaemonManagerConfigUsesEnvOverride (verifies whitespace-trimmed env override is applied)
• TestDaemonManagerConfigFallsBackToDefaultRegistry (verifies default remains version.DockerRegistry when unset)

Validation

• go test ./pkg/cli/...
• make build-cli
• ./bin/arctl daemon start

./bin/arctl daemon start and ARCTL_DAEMON_DOCKER_REGISTRY=registry.example.com ./bin/arctl daemon start are blocked in this environment because Docker daemon is not running, so both fail with:
Cannot connect to the Docker daemon at ... before registry resolution is reached.

Issue

Fixes #553

Known limitations

• This change adds environment support but does not add a dedicated CLI flag for override; only ARCTL_DAEMON_DOCKER_REGISTRY is supported in this patch.