Temporary Dockerfile sometimes fails with permission issues

[robdimsdale]

On some machines (but not all) we noticed that bookdocker fails with permissions error:

 => ERROR [internal] load build definition from Dockerfile                                                         0.0s
 => => transferring dockerfile: 106B                                                                               0.0s
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 > [internal] load build definition from Dockerfile:
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failed to solve with frontend dockerfile.v0: failed to read dockerfile: error from sender: open /private/tmp/KSOutOfProcessFetcher.0x6uzinfjm: permission denied

The way we solved this was changing the way docker reads in the Dockerfile as follows:

docker build --tag "${image_tag}" --file - . < /tmp/Dockerfile

(See: https://github.com/docker/cli/issues/2249 for more info).

This change seems to fix the issue on the machines where it didn't work, and doesn't seem to impact machines where it previously worked.

Would you like a PR for this?