use block syntax for defaults

[pchalamet]

simplify syntax for extension defaults.

old syntax:

extension @docker {
    defaults = {
        image: "ghcr.io/magnusopera/dotnet-app"
        arguments: { configuration: $configuration }
    }
}

new syntax:

extension @docker {
    defaults {
        `image` = "ghcr.io/magnusopera/dotnet-app"
        arguments = { configuration: $configuration }
    }
}