Update building doc for Docker for Mac users

Signed-off-by: Jess Valarezo <valarezo.jessica@gmail.com>

BUILDING.md

@@ -45,6 +45,8 @@ sudo make install
 
 ## Via Docker Container
 
+### Build containerd
+
 You can build `containerd` via Docker container. You can build an image from
 this `Dockerfile`:
 
@@ -71,3 +73,61 @@ You can move the binaries in your `$PATH` with the command:
 ```sh
 sudo make install
 ```
+
+### Build runc and containerd
+
+To have complete core container runtime, you will need both `containerd` and `runc`. It is possible to build both of these via Docker container.
+
+You can use `go` to checkout `runc` in your `GOPATH`:
+
+```sh
+go get github.com/opencontainers/runc
+```
+
+We can build an image from this `Dockerfile`
+
+```sh
+FROM golang
+
+RUN apt-get update && \
+    apt-get install -y btrfs-tools libapparmor-dev libseccomp-dev
+
+```
+
+In our Docker container we will use a specific `runc` build which includes [seccomp](https://en.wikipedia.org/wiki/seccomp) and [apparmor](https://en.wikipedia.org/wiki/AppArmor) support. Hence why our Dockerfile includes these dependencies: `libapparmor-dev` `libseccomp-dev`.
+
+Let's suppose you build an image called `containerd/build` from the above Dockerfile. You can run the following command:
+
+```sh
+docker run -it --privileged \
+    -v /var/lib/containerd \
+    -v ${GOPATH}/src/github.com/opencontainers/runc:/go/src/github.com/opencontainers/runc \
+    -v ${GOPATH}/src/github.com/containerd/containerd:/go/src/github.com/containerd/containerd \
+    -e GOPATH=/go
+    -w /go/src/github.com/containerd/containerd containerd/build sh
+```
+
+This mounts both `runc` and `containerd` repositories in our Docker container.
+
+From within our Docker container let's build `containerd`
+
+```sh
+cd /go/src/github.com/containerd/containerd
+make && make install
+```
+
+These binaries can be found in the `./bin` directory in your host.
+`make install` will move the binaries in your `$PATH`.
+
+Next, let's build `runc`
+
+```sh
+cd /go/src/github.com/opencontainers/runc
+make BUILDTAGS='seccomp apparmor' && make install
+```
+
+When working with `ctr`, the containerd CLI we just built, don't forget to start `containerd`!
+
+```sh
+containerd --config config.toml
+```

RUNC.md

@@ -1,11 +1,17 @@
-containerd is built with OCI support and with support for advanced features provided by `runc`.
+containerd is built with OCI support and with support for advanced features provided by [runc](https://github.com/opencontainers/runc).
 
-We depend on a specific runc version when dealing with advanced features.  You should have a specific build for development.  The current supported runc commit is:
+We depend on a specific `runc` version when dealing with advanced features.  You should have a specific runc build for development.  The current supported runc commit is:
 
 RUNC_COMMIT = e775f0fba3ea329b8b766451c892c41a3d49594d
 
+For more information on how to clone and build runc see the runc Building [documentation](https://github.com/opencontainers/runc#building).
+
+Note: before building you may need to install additional support, which will vary by platform. For example, you may need to install `libseccomp` and `libapparmor` e.g. `libseccomp-dev` and `libapparmor-dev` for Ubuntu.
+
 ## building
 
+From within your `opencontainers/runc` repository run:
+
 ### apparmor
 
 ```bash