fix docker SwiftPM cache setup (#3429)
fix docker SwiftPM cache setup (#3429)

motivation: fixed shared repositories cache between mac and linux using docker

changes: since ~/.swiftpm/cache is a symlink, define the volume mount there instead of the parent directory
apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager