fix: fix TLS Certificate Verification Failure for Private Registry Artifact Download

fix: fix TLS Certificate Verification Failure for Private Registry Artifact Download

Root Cause: The agent fails to download the Trivy vulnerability database artifact because the TLS certificate chain for the private registry 'registry.dev.nudgebee.pollux.in' is not trusted by the execution environment (`x509: certificate signed by unknown authority`).

Proposed Fix: The fix involves modifying the agent's entry point (`cmd/agent/main.go`) to check for a custom CA certificate bundle path provided via the environment variable `AGENT_CUSTOM_CA_BUNDLE_PATH`. If present, this path is used to configure the Go runtime's TLS system to trust the registry's certificate authority before any network operations (like DB download) occur. This is the standard production pattern for handling private registry TLS issues in Go applications.

Impact: Resolves the fatal error during agent initialization caused by untrusted TLS certificates when downloading the vulnerability database from the private registry, ensuring successful startup and operation.

## Original Error/Logs

<details>
<summary>Click to expand the original error that triggered this fix</summary>

```
2026-01-24T10:30:53.932221503Z 2026-01-24T10:30:53Z	FATAL	Fatal error	init error: DB error: failed to download vulnerability DB: OCI artifact error: failed to download vulnerability DB: failed to download artifact from registry.dev.nudgebee.pollux.in/trivy-db:latest: OCI repository error: 1 error occurred:
2026-01-24T10:30:53.932260845Z 	* Get "https://registry.dev.nudgebee.pollux.in/v2/": tls: failed to verify certificate: x509: certificate signed by unknown authority
2026-01-24T10:30:53.932265529Z
2026-01-24T10:30:53.932267581Z
```

</details>

---

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Author: nudgebee-bot

cmd/agent/main.go

@@ -13,6 +13,8 @@ limitations under the License.
 package main
 
 import (
+	"os"
+	"os/signal"
 	"fmt"
 	"os"
 	"os/signal"