test(ci): container image smoke test via testcontainers

[Prarthana124]

What

Adds a Docker image integration smoke test using testcontainers.

The test builds and runs the freshly built Docker image and verifies:

• Container starts successfully
• kerno version executes correctly
• kerno --help exits successfully
• Container lifecycle is handled cleanly

Why

The Docker CI job currently builds the image but does not verify that the image can actually run.
This integration test helps catch broken entrypoints, missing binaries, and invalid default commands before release.
Fixes #147

How

• Added a new integration test in internal/integration/container_smoke_test.go
• Uses testcontainers-go to run the freshly built Docker image
• Verifies container startup and basic CLI commands
• Ensures the container exits and cleans up correctly

Testing

• go build ./... passes
• go test ./... passes
• go vet ./... passes
• golangci-lint run ./... passes
• Tested locally with:

• N/A — pure docs/refactor
• sudo ./bin/bpf-verify --read 5s confirms 6/6 programs still load
• ./scripts/verify.sh passes (or specific phase: ./scripts/verify.sh quality)

Checklist

• PR title follows Conventional Commits (feat(scope): subject)
• All commits are DCO-signed (git commit -s)
• No unrelated changes pulled in
• Documentation updated where user-visible behavior changed
• Added/updated tests for new code paths
• If a new doctor rule, paired with a chaos scenario in scripts/verify.sh

[github-actions]

🚀 First PR — welcome aboard!

A few things to expect:

1. CI: every PR runs build + race tests + lint + (eventually) the kernel matrix. If something fails, the log will tell you exactly which gate.
2. DCO: every commit needs Signed-off-by: — git commit -s adds it automatically.
3. Conventional Commits: PR titles like feat(doctor): add new rule or fix(bpf): handle X. We squash-merge by default.
4. Review: a maintainer will review within 72 hours. Suggestions are conversations, not orders — push back if something doesn't fit your context.

If you get stuck, reply here or jump to Discussions. We want this PR to land.