build: add syntax directives to dockerfile examples

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

build/building/cache/index.md

@@ -22,6 +22,7 @@ Have a look at the following example, which shows a simple Dockerfile for a
 program written in C.
 
 ```dockerfile
+# syntax=docker/dockerfile:1
 FROM ubuntu:latest
 
 RUN apt-get update && apt-get install -y build-essentials
@@ -86,6 +87,7 @@ Consider the following example. A Dockerfile snippet that runs a JavaScript
 build from the source files in the current directory:
 
 ```dockerfile
+# syntax=docker/dockerfile:1
 FROM node
 WORKDIR /app
 COPY . .          # Copy over all files in the current directory
@@ -103,6 +105,7 @@ the dependencies. Finally, copy over the project source code, which is subject
 to frequent change.
 
 ```dockerfile
+# syntax=docker/dockerfile:1
 FROM node
 WORKDIR /app
 COPY package.json yarn.lock .    # Copy package management files
@@ -225,6 +228,8 @@ illustrates building a simple web server that serves HTML from your `docs`
 directory in Git:
 
 ```dockerfile
+# syntax=docker/dockerfile:1
+
 # stage 1
 FROM alpine as git
 RUN apk add git

build/building/context.md

@@ -41,6 +41,7 @@ $ docker build .
 With the following Dockerfile:
 
 ```dockerfile
+# syntax=docker/dockerfile:1
 FROM busybox
 WORKDIR /src
 COPY foo .
@@ -64,6 +65,7 @@ files required by the `COPY` instructions, in this case `foo`.
 In some cases you may want to send the entire context:
 
 ```dockerfile
+# syntax=docker/dockerfile:1
 FROM busybox
 WORKDIR /src
 COPY . .

build/buildkit/configure.md

@@ -148,6 +148,8 @@ rel="noopener" class="_"} in BuildKit as an example. Feel free to include your
 own CNI configuration.
 
 ```dockerfile
+# syntax=docker/dockerfile:1
+
 ARG BUILDKIT_VERSION=v{{ site.buildkit_version }}
 ARG CNI_VERSION=v1.0.1
 

build/buildx/install.md

@@ -20,6 +20,7 @@ Here is how to install and use Buildx inside a Dockerfile through the
 [`docker/buildx-bin`](https://hub.docker.com/r/docker/buildx-bin) image:
 
 ```dockerfile
+# syntax=docker/dockerfile:1
 FROM docker
 COPY --from=docker/buildx-bin:latest /buildx /usr/libexec/docker/cli-plugins/docker-buildx
 RUN docker buildx version