feat: add custom health check commands per app

Add configurable health check commands per app via a new
'healthcheck_command' field. When set, the command is passed
to Docker as a CMD-SHELL health check on the container.
When empty, the image's default health check is used.

Changes:
- Add migration 007 for healthcheck_command column on apps table
- Add HealthcheckCommand field to App model with full CRUD support
- Add buildHealthcheck() to docker client for CMD-SHELL config
- Pass health check command through CreateContainerOptions
- Add health check command input to app create/edit UI forms
- Extract optionalNullString helper to reduce handler complexity
- Update README features list

closes #81

internal/docker/client.go

@@ -13,6 +13,7 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"time"
 
 	dockertypes "github.com/docker/docker/api/types"
 	"github.com/docker/docker/api/types/container"
@@ -138,13 +139,14 @@ func (c *Client) BuildImage(
 
 // CreateContainerOptions contains options for creating a container.
 type CreateContainerOptions struct {
-	Name    string
-	Image   string
-	Env     map[string]string
-	Labels  map[string]string
-	Volumes []VolumeMount
-	Ports   []PortMapping
-	Network string
+	Name               string
+	Image              string
+	Env                map[string]string
+	Labels             map[string]string
+	Volumes            []VolumeMount
+	Ports              []PortMapping
+	Network            string
+	HealthcheckCommand string // Custom health check shell command (empty = use image default)
 }
 
 // VolumeMount represents a volume mount.
@@ -185,6 +187,29 @@ func buildPortConfig(ports []PortMapping) (nat.PortSet, nat.PortMap) {
 	return exposedPorts, portBindings
 }
 
+// healthcheckInterval is the time between health check attempts.
+const healthcheckInterval = 30 * time.Second
+
+// healthcheckTimeout is the maximum time a single health check can take.
+const healthcheckTimeout = 10 * time.Second
+
+// healthcheckStartPeriod is the grace period before health checks start counting failures.
+const healthcheckStartPeriod = 15 * time.Second
+
+// healthcheckRetries is the number of consecutive failures needed to mark unhealthy.
+const healthcheckRetries = 3
+
+// buildHealthcheck creates a Docker health check config from a shell command string.
+func buildHealthcheck(command string) *container.HealthConfig {
+	return &container.HealthConfig{
+		Test:        []string{"CMD-SHELL", command},
+		Interval:    healthcheckInterval,
+		Timeout:     healthcheckTimeout,
+		StartPeriod: healthcheckStartPeriod,
+		Retries:     healthcheckRetries,
+	}
+}
+
 // CreateContainer creates a new container.
 func (c *Client) CreateContainer(
 	ctx context.Context,
@@ -218,14 +243,22 @@ func (c *Client) CreateContainer(
 	// Convert ports to exposed ports and port bindings
 	exposedPorts, portBindings := buildPortConfig(opts.Ports)
 
+	// Build container config
+	containerConfig := &container.Config{
+		Image:        opts.Image,
+		Env:          envSlice,
+		Labels:       opts.Labels,
+		ExposedPorts: exposedPorts,
+	}
+
+	// Apply custom health check if configured
+	if opts.HealthcheckCommand != "" {
+		containerConfig.Healthcheck = buildHealthcheck(opts.HealthcheckCommand)
+	}
+
 	// Create container
 	resp, err := c.docker.ContainerCreate(ctx,
-		&container.Config{
-			Image:        opts.Image,
-			Env:          envSlice,
-			Labels:       opts.Labels,
-			ExposedPorts: exposedPorts,
-		},
+		containerConfig,
 		&container.HostConfig{
 			Mounts:       mounts,
 			PortBindings: portBindings,