desktop: add syntax directives to dockerfile examples

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

desktop/containerd/index.md

@@ -152,6 +152,7 @@ You can also check from the browser that Nginx is running:
 Sample Dockerfile:
 
 ```dockerfile
+# syntax=docker/dockerfile:1
 FROM alpine
 
 ENTRYPOINT ["echo", "hello friends"]

desktop/dev-environments/set-up.md

@@ -63,6 +63,7 @@ In the yaml file, the build context `backend` specifies that that the container
 The `development` stage of the Dockerfile is defined as follows:
 
 ```dockerfile
+# syntax=docker/dockerfile:1
 FROM golang:1.16-alpine AS build
 WORKDIR /go/src/github.com/org/repo
 COPY . .

desktop/wasm/index.md

@@ -151,6 +151,7 @@ running in a container.
    the `ENTRYPOINT`.
 
    ```dockerfile
+   # syntax=docker/dockerfile:1
    FROM scratch
    COPY --from=build /build/hello_world.wasm /hello_world.wasm
    ENTRYPOINT [ "hello_world.wasm" ]