fix instructions for registry.json
fix instructions for registry.json

The previous instructions fixed situations where the directory
and file didn't exist, but had some issues remaining;

The `>>` _appends_ the content to the file; this would work if the file didn't
exist (before `touch`'ing it) but if there happened to be a file already, it
would append the content to the existing file, resulting in invalid JSON.

e.g. running it twice (which may be accidentally);

```bash
sudo echo '{"allowedOrgs":["myorg"]}' >> "/Library/Application Support/com.docker.docker/registry.json"
sudo echo '{"allowedOrgs":["myorg"]}' >> "/Library/Application Support/com.docker.docker/registry.json"
```

Would result in;

```bash
sudo cat "/Library/Application Support/com.docker.docker/registry.json"
{"allowedOrgs":["myorg"]}
{"allowedOrgs":["myorg"]}
```

(which is invalid JSON)

The `sudo echo` also had some issues; the `sudo` only applied to the `echo`, and
not to the piped output (see https://unix.stackexchange.com/questions/1416/redirecting-stdout-to-a-file-you-dont-have-write-permission-on);

```bash
$ sudo touch "/Library/Application Support/com.docker.docker/registry.json"
$ sudo echo '{"allowedOrgs":["myorg"]}' > "/Library/Application Support/com.docker.docker/registry.json"
bash: /Library/Application Support/com.docker.docker/registry.json: Permission denied
```

Instead, using `tee` to run as privileged process, similar to the approach taken
in https://github.com/docker/docker.github.io/blame/c33d39a7ba80ad86e3d6f1bd38f21e530d58819e/engine/install/debian.md#L106

With that, the `touch` wouldn't be needed (the `mkdir` still would though);

```suggestion
$ sudo mkdir -p "/Library/Application Support/com.docker.docker"
$ echo '{"allowedOrgs":["myorg"]}' | sudo tee "/Library/Application Support/com.docker.docker/registry.json"
```

The new instructions will always overwrite the content with the new content,
replacing what's already there (that may need a small warning / note though) to
prevent the invalid JSON if someone runs the command multiple times, or if they
have to update the existing config with a new one.

Finally, some verification steps were added to verify content and permissions.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

thaJeztah/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io