feat(docker-watcher): phase 6 - webhook handler

Secret UUID-based webhook endpoint for CI image push notifications.
Project/stage matching via glob patterns, auto-creation of unknown
projects from image inspection. Fix JSON response injection.

internal/health/checker.go

@@ -0,0 +1,79 @@
+package health
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"time"
+)
+
+// DefaultRetries is the number of health check attempts before declaring failure.
+const DefaultRetries = 3
+
+// DefaultRetryInterval is the pause between health check retries.
+const DefaultRetryInterval = 5 * time.Second
+
+// DefaultTimeout is the HTTP timeout for a single health check attempt.
+const DefaultTimeout = 10 * time.Second
+
+// Checker performs HTTP health checks against a container endpoint.
+type Checker struct {
+	httpClient    *http.Client
+	retries       int
+	retryInterval time.Duration
+}
+
+// New creates a Checker with default settings.
+func New() *Checker {
+	return &Checker{
+		httpClient: &http.Client{
+			Timeout: DefaultTimeout,
+		},
+		retries:       DefaultRetries,
+		retryInterval: DefaultRetryInterval,
+	}
+}
+
+// Check performs an HTTP GET health check against the given URL.
+// It retries up to the configured number of times, waiting retryInterval between attempts.
+// Returns nil on the first successful (2xx) response, or the last error encountered.
+func (c *Checker) Check(ctx context.Context, url string) error {
+	var lastErr error
+
+	for attempt := 0; attempt < c.retries; attempt++ {
+		if attempt > 0 {
+			select {
+			case <-ctx.Done():
+				return fmt.Errorf("health check cancelled: %w", ctx.Err())
+			case <-time.After(c.retryInterval):
+			}
+		}
+
+		lastErr = c.doCheck(ctx, url)
+		if lastErr == nil {
+			return nil
+		}
+	}
+
+	return fmt.Errorf("health check failed after %d attempts: %w", c.retries, lastErr)
+}
+
+// doCheck performs a single HTTP GET health check.
+func (c *Checker) doCheck(ctx context.Context, url string) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return fmt.Errorf("create health check request: %w", err)
+	}
+
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return fmt.Errorf("health check request to %s: %w", url, err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return fmt.Errorf("health check %s returned status %d", url, resp.StatusCode)
+	}
+
+	return nil
+}