https://docs.docker.com/engine/reference/commandline/swarm/

Explanation
The PVC remains unbound until a persistentVolume that matches all requirements of the PVC becomes available is an action Kubernetes takes in this situation. A persistentVolumeClaim (PVC) is a request for storage by a user. A PVC specifies the desired size, access modes, and storage class of the storage. A persistentVolume (PV) is a piece of storage in the cluster that has been provisioned by an administrator. A PV has a lifecycle independent of any individual pod that uses it. A PVC is automatically bound to a suitable PV based on its requirements. If no PV exists that matches the PVC, and dynamic provisioning is not enabled, then the PVC remains unbound indefinitely until a matching PV is created. References:
https://kubernetes.io/docs/concepts/storage/persistent-volumes/,
https://kubernetes.io/docs/concepts/storage/persistent-volumes/#lifecycle-of-a-volume-and-claim

Explanation
= The command 'docker service create -name dns-cache -p 53:53/udp dns-cache' is not correct and will not create a swarm service that only listens on port 53 using the UDP protocol. There are two errors in the command:
* The option -name should be --name with two dashes, otherwise it will be interpreted as a short option -n followed by an argument ame1.
* The option -p or --publish will publish the service port to the host port, which means the service will be reachable from outside the swarm2. To create a service that only listens on the internal network, you need to use the --publish-add option with the mode=ingress flag3.
The correct command should be:
docker service create --name dns-cache --publish-add mode=ingress,target=53,published=53,protocol=udp dns-cache References:
* docker service create | Docker Docs
* Publish ports on the host | Docker Docs
* Publish a port for a service | Docker Docs

The command docker service create -name dns-cache -p 53:53 -constraint networking.protocol.udp=true dns-cache will not create a swarm service that only listens on port 53 using the UDP protocol. This command has several syntax errors and invalid options. The correct syntax for creating a swarm service is docker service create [OPTIONS] IMAGE [COMMAND] [ARG...]1. The correct options for specifying the service name, port mapping, and network mode are --name, --publish, and --network respectively1. The option -constraint is not a valid option for the docker service create command. To create a swarm service that only listens on port 53 using the UDP protocol, you need to use the --publish option with the protocol=udp and mode=host parameters, and the --network option with the host value23. For example, the following command creates a global service using host mode and bypassing the routing mesh2:
docker service create --name dns-cache \
--publish published=53,target=53,protocol=udp,mode=host \
--mode global \
--network host \
dns-cache
:
1: docker service create | Docker Docs
2: Use swarm mode routing mesh | Docker Docs
3: Manage swarm service networks | Docker Docs

Docker Content Trust (DCT) is a feature that allows users to verify the integrity and publisher of container images they pull or deploy from a registry server, signed on a Notaryserver1. DCT is enabled by setting the environment variable DOCKER_CONTENT_TRUST=1 on the Docker client. When DCT is enabled, the Docker client will only pull, run, or build images that have valid signatures for a specific tag2. However, DCT does not apply to the docker image import command, which allows users to import an image or a tarball with a repository and tag from a file or STDIN3. Therefore, if myorg/myimage:1.0 is unsigned, Docker will not block the docker image import <tarball>myorg/myimage:1.0 command, even if DCT is enabled. This is because the docker image import command does not interact with a registry or a Notary server, and thus does not perform any signature verification. However, this also means that the imported image will not have any trust data associated with it, and it will not be possible to push it to a registry with DCT enabled, unless it is signed with a valid key. Reference:
Content trust in Docker
Automation with content trust
[docker image import]
[Content trust and image tags]