Race condition caused by not using image hash

- docker-plugin: 1.1.6
- jenkins: 2.214

Having this as part of declarative pipeline:
```
image = docker.build("project", "-f Dockerfile .")
image.push("version")
```

Current behavior essentially translates to:
```bash
docker build -t project -f Dockerfile .
docker tag project project:version
docker push project:version
```
If we expand defaults a bit it becomes:
```bash
docker build -t project:latest -f Dockerfile .
docker tag project:latest project:version
docker push project:version
```

The expected behavior however would be this:
```bash
image_hash=$(docker build -t project:latest -f Dockerfile -q .)
docker tag $image_hash project:version
docker push project:version
```

Rationale:
Current behavior is confusing because it allows race condition and you may end up with `project:version` image that doesn't correspond to the source code you were building.

Yes, there are easy ways to work around this by specifying tag explicitly, but still I think `image` entity in the pipeline should represent image that was just produced by its unique immutable hash.