Fix Armv7 Docker Build

[bauerj]

Another attempt at #145 😅

This time with help from @schnuffle.

Upstream pikepdf provides binary wheels for amd64 and arm64 but not armv7. This means we have to compile it on that architecture. This also means that we have to compile its dependency qpdf manually.

This takes almost forever (two hours!) but with improved Docker caching, this should hopefully be not much of an issue.

Closes: #150

[coveralls]

Pull Request Test Coverage Report for Build 1903099095

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.01%

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Totals
Change from base Build 1901054761:	0.0%
Covered Lines:	3950
Relevant Lines:	4293

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💛 - Coveralls

[bauerj]

Thanks to the both of you!

Some further remarks on Docker caching:

The currently used strategy local has a warning that "At the moment caches are copied over the existing cache so it keeps growing." This should not happen with a gha-type cache.

[schnuffle]

One minor note, in my POC I've only pulled qpdf and did not checkout a distinct version.

Okay, checked the latest state and somebody else has been faster.

Good job

[qcasey]

#150 (comment)

[github-actions]

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