Fix compile error preventing Docker image to be built

I tried to test the test runner using the following command:

```
./bin/run-in-docker.sh multiple-single-fail /home/erik/exercism-repos/swift-test-runner/test/multiple-tests-single-fail /home/erik/exercism-repos/swift-test-runner/test/multiple-tests-single-fail
```

This results in a compile error:

```
> [builder 4/4] RUN swift build --configuration release:
#11 2.685 Fetching https://github.com/apple/swift-argument-parser
#11 4.763 Cloning https://github.com/apple/swift-argument-parser
#11 4.816 Resolving https://github.com/apple/swift-argument-parser at 0.4.1
#11 23.95 [1/2] Compiling ArgumentParser BashCompletionsGenerator.swift
#11 24.56 [2/3] Compiling TestRunner Parser.swift
#11 24.56 /Sources/TestRunner/main.swift:20:4: error: 'init()' is unavailable: A default value must be provided unless the value type is supported by Flag.
#11 24.56 @Flag()
#11 24.56 ^~~~
#11 24.56 /.build/checkouts/swift-argument-parser/Sources/ArgumentParser/Parsable Properties/Flag.swift:62:10: note: 'init()' has been explicitly marked unavailable here
#11 24.56 public init() {
#11 24.56 ^
------
executor failed running [/bin/sh -c swift build --configuration release]: exit code: 1
```