stack deploy: fail early on invalid image reference format

Before this patch, `docker stack deploy` would not validate the image
reference on the client side, depending on the daemon to return an error,
which was not always easy to interpret;

    docker stack deploy -c docker-compose.yaml mystack
    Creating service mystack_myservice
    failed to create service mystack_myservice: Error response from daemon: rpc error: code = InvalidArgument desc = ContainerSpec: image reference must be provided

    IMAGE_NAME=FOOBAR  docker stack deploy -c docker-compose.yaml mystack
    Creating service mystack_myservice
    failed to create service mystack_myservice: Error response from daemon: rpc error: code = InvalidArgument desc = ContainerSpec: "FOOBAR" is not a valid repository/tag

With this patch, the CLI validates the image-reference for each service,
producing an error if the reference is empty or invalid.

    docker stack config -c docker-compose.yaml
    invalid service myservice: no image specified

    IMAGE_NAME=FOOBAR  ~/Projects/cli/build/docker stack deploy -c docker-compose.yaml mystack
    invalid image reference for service myservice: invalid reference format: repository name (library/FOOBAR) must be lowercase

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

cli/command/stack/loader/loader.go

@@ -12,6 +12,7 @@ import (
 	"sort"
 	"strings"
 
+	"github.com/distribution/reference"
 	"github.com/docker/cli/cli/command"
 	"github.com/docker/cli/cli/command/stack/options"
 	"github.com/docker/cli/cli/compose/loader"
@@ -49,6 +50,17 @@ func LoadComposefile(dockerCli command.Cli, opts options.Deploy) (*composetypes.
 		_, _ = fmt.Fprintf(dockerCli.Err(), "Ignoring deprecated options:\n\n%s\n\n",
 			propertyWarnings(deprecatedProperties))
 	}
+
+	// Validate if each service has a valid image-reference.
+	for _, svc := range config.Services {
+		if svc.Image == "" {
+			return nil, errors.Errorf("invalid image reference for service %s: no image specified", svc.Name)
+		}
+		if _, err := reference.ParseAnyReference(svc.Image); err != nil {
+			return nil, errors.Wrapf(err, "invalid image reference for service %s", svc.Name)
+		}
+	}
+
 	return config, nil
 }