Update "gpg" invocations to use "--batch"

[tianon]

See https://bugs.debian.org/913614

[CLAassistant]

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[lag-linaro]

I was about to submit this myself.

LGTM.

[lag-linaro]

@tianon looks like you have to sign the agreement to contribute.

[tianon]

That seems kind of silly given that I already have merge access and previous commits. I'm a bit concerned about the maintenance of this image, to be honest (doesn't seem to be a whole lot of activity on this repository besides the bot doing version bumps).

[lag-linaro]

Maybe @pierreozoux, @sampaiodiego or @geekgonecrazy can shed some light?

[pierreozoux]

@lag-linaro see #61

[lag-linaro]

@lag-linaro see #61

I see it. Is there anything you want me to action? Or just be aware of it?

Will someone review my PR too? #60