Add Swift devcontainer template (devcontainers#150)

Author: adam-fowler

_data/collection-index.yml

@@ -277,4 +277,9 @@
   maintainer: r3dpoint
   contact: https://github.com/r3dpoint/devcontainer-features/issues
   repository: https://github.com/r3dpoint/devcontainer-features
-  ociReference: ghcr.io/r3dpoint/devcontainer-features
+  ociReference: ghcr.io/r3dpoint/devcontainer-features
+- name: Swift Container Templates
+  maintainer: Swift Server Workgroup
+  contact: https://github.com/swift-server/swift-devcontainer-template/issues
+  repository: https://github.com/swift-server/swift-devcontainer-template
+  ociReference: ghcr.io/swift-server/swift-devcontainer-template