Add an example of building an application and running in Docker or LinuxKit

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

examples/README.md

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+Examples of building an image to run on LinuxKit or on a host
+
+Currently the `moby` tool can output formats suitable for LinuxKit to boot on
+a VM, and also some formats for running natively.
+
+The `docker` format adds a `Dockerfile` to the tarball and expects a similar
+file structure to `LinuxKit` but with the low level system setup pieces removed
+as this will already be set up in a container.
+
+The `mobytest/init-container` image in this repository has an example setup that
+initialises `containerd` in exactly the same way as it runs in LinuxKit, which will
+then start the `onboot` and `service` containers. The example below shows how you
+can run `nginx` on either of these base configs.
+
+```
+moby build -output docker -o - docker.yml nginx.yml | docker build -t dockertest -
+docker run -d -p 80:80 --privileged dockertest
+
+moby build -output kernel+initrd linuxkit.yml nginx.yml
+linuxkit run nginx
+```
+
+Both of these will run the same `nginx` either in a VM or a container.

examples/docker.yml

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+init:
+  - mobytest/init-container:b5de246b2790d74d53bae583a95c87869ca003e6
+  - linuxkit/runc:3a4e6cbf15470f62501b019b55e1caac5ee7689f
+  - linuxkit/containerd:5749f2e9e65395cc6635229e8da0e0d484320ddf
+  - linuxkit/ca-certificates:75cf419fb58770884c3464eb687ec8dfc704169d
+trust:
+  org:
+    - linuxkit
+    - mobytest

examples/linuxkit.yml

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+kernel:
+  image: "linuxkit/kernel:4.9.x"
+  cmdline: "console=ttyS0"
+init:
+  - linuxkit/init:1b8a7e394d2ec2f1fdb4d67645829d1b5bdca037
+  - linuxkit/runc:3a4e6cbf15470f62501b019b55e1caac5ee7689f
+  - linuxkit/containerd:5749f2e9e65395cc6635229e8da0e0d484320ddf
+  - linuxkit/ca-certificates:75cf419fb58770884c3464eb687ec8dfc704169d
+onboot:
+  - name: dhcpcd
+    image: "linuxkit/dhcpcd:7d2b8aaaf20c24ad7d11a5ea2ea5b4a80dc966f1"
+    command: ["/sbin/dhcpcd", "--nobackground", "-f", "/dhcpcd.conf", "-1"]
+services:
+  - name: rngd
+    image: "linuxkit/rngd:1fa4de44c961bb5075647181891a3e7e7ba51c31"
+trust:
+  org:
+    - linuxkit

examples/nginx.yml

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+services:
+  - name: nginx
+    image: "nginx:alpine"
+    capabilities:
+     - CAP_NET_BIND_SERVICE
+     - CAP_CHOWN
+     - CAP_SETUID
+     - CAP_SETGID
+     - CAP_DAC_OVERRIDE
+trust:
+  org:
+    - library