Documentation missing: How to run a local registry? (additionally I get an error)

The documentation and the conference videos don't show how to run a local registry which swift-container-plugin needs.

From googling and from issue #68 I was able to figure out that running

`docker run -d -p 5555:5000 registry:2`

should do the trick.

Unfortunately I get the following error when swift-container-plugin runs:

```
swift run containertool .build/x86_64-swift-linux-musl/debug/helloServer --repository localhost:5555/hello -v
Building for debugging...
[1/1] Write swift-version--58304C5D6DBC2206.txt
Build of product 'containertool' complete! (0.26s)
Base image architecture: amd64
Connected to source registry: index.docker.io
Connected to destination registry: localhost:5555
Using base image: index.docker.io/library/swift:slim
Found base image manifest: sha256:bcbe7e057d0a9a9685bce1917efb8fd3fe350a28e6f1b8184989553933e1bc96
Found base image configuration: sha256:8dae9dd90c7bc6eefac511b3fe38e2238741fcd90327640cd4a0397139cc1773
Error: dataCorrupted(Swift.DecodingError.Context(codingPath: [], debugDescription: "The given data was not valid JSON.", underlyingError: Optional(Error Domain=NSCocoaErrorDomain Code=3840 "Unexpected character 'p' after top-level value around line 1, column 5." UserInfo={NSJSONSerializationErrorIndex=4, NSDebugDescription=Unexpected character 'p' after top-level value around line 1, column 5.})))
```