Introduce image digest pinning in Portainer

[deviantony]

Following the merge of #1984

This PR introduced a work-around inside Portainer to leverage image digest pinning on the daemon side. This is not supported for the recent Docker client versions and I believe that we should follow how the Docker Client interacts with the Docker daemon.

Thus, we should implement digest pinning inside Portainer, e.g. the ability to query a registry to retrieve the digest associated to an image repository before sending a create/update service request (and remove the work-around introduced by the aforementioned PR by the way).

This will give us control over any error that might be raised during the retrieval of the digest associated to an image, which is not the case at the moment as it's done inside the Docker daemon.

[xAt0mZ]

As #cebb5a is removing the code comment on the workaround for digest pinning, I'm documenting it.

Code comment:

This is a temporary work-around that allows us to leverage digest pinning
on the Docker daemon side.
It has been moved client-side since Docker API version > 1.29.
We should introduce digest pinning in Portainer as well.

Explanation:
We are adding the header version: 1.29 to services create and update requests to leverage digest pinning on the Docker daemon side.

    create: {
      method: 'POST', params: {action: 'create'},
      headers: {
        'X-Registry-Auth': HttpRequestHelper.registryAuthenticationHeader,
        'version': '1.29'
      },
      ignoreLoadingBar: true
    },
    update: {
      method: 'POST', params: { id: '@id', action: 'update', version: '@version', rollback: '@rollback' },
      headers: {
        'version': '1.29'
      }
    },