Fix panic when Docker is unresponsive (Issue #312)

[aviralgarg05]

Related issue
Fixes #312

What I did
When Docker Desktop is unresponsive (e.g., paused or exhausted resources), the internal apiClient() wrapper in pkg/docker returns nil. Previously, the code did not check for this, leading to a nil pointer dereference panic when tools like mcp-add or mcp-find attempted to interact with Docker.

This PR introduces a safe client() helper method in pkg/docker/client.go that checks if the API client is nil and returns a descriptive error ("docker client is not available") instead of panicking.

I have updated all files in pkg/docker (containers.go, images.go, network.go, volumes.go) to use this safe helper.

I also added a regression test TestClientSafeError in pkg/docker/client_test.go that simulates a nil client and verifies that:

1. The application does not panic.
2. The correct error message is returned.

Verification

• Run go test -v ./pkg/docker/... to verify the fix and ensure no regressions.
• Manual verification: The gateway should now return a graceful error message to the client instead of crashing if Docker is unavailable.

(not mandatory) A picture of a cute animal

[Pnkcaht]

Almost perfect, but with a critical bug in ImageExists: it uses the ContainerInspect API instead of ImageInspect, which I think is wrong (containers ≠ images)

[Pnkcaht]

I don't yet have permission to actually approve and merge it, but I believe the fix you implemented will go straight to the merge by the team. Anyway, LGTM for me, good pull request