[Format] Allow overriding the docker container used for swift-format

[fabianfett]

github-workflows/.github/workflows/soundness.yml

Line 150 in 66ec6fb

image: swift:6.0-jammy

We should allow users to override the docker container that is used for running the format soundness check.

Reason

Swift 6.0 does not understand the sending keyword. swiftlang/swift-syntax#2836 We would like to force the 6.0 nightly in our project.

See open PR that can't use the workflows defined here:
swift-server/swift-aws-lambda-runtime#405

[FranzBusch]

Done in #21