Mirror official images to `swiftlang/swift`

[rauhul]

We should include copies of the official images under swiftlang/swift so folks writing docker files/actions/etc that need to work pre-release version don't need to find images from both swift and swiftlang/swift.

[rauhul]

Forum discussion: Why are docker images split between two orgs?

[shahmishal]

Workaround for GitHub Actions:

container: ${{ (contains(matrix.swift_version, 'nightly') && 'swiftlang/swift') || 'swift' }}:${{ matrix.swift_version }}