Add WP_CLI_ALLOW_ROOT env var to Dockerfile

[tbradsha]

We set WP_CLI_ALLOW_ROOT in the default.env file, but directly using the container would bypass that file (e.g. in our Post Build tests). This PR adds that env var to the Dockerfile.

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After merge, verify "Post Build tests" complete as expected.

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[anomiex]

While this might be needed too, we definitely need it in tools/docker/Dockerfile.

(this tools/docker/Dockerfile.monorepo is for the projects/js-packages/jetpack-cli/ stuff)

[anomiex]

On second look, probably tools/docker/Dockerfile.monorepo doesn't need it. It doesn't seem to contain an install of WordPress at all.

[anomiex]

LGTM!