Work around Synapse Docker build failures by using a --cache-dir

[squahtx]

When the --no-cache-dir option is given to pip, setuptools>=48.0.0
installs msgpack to /usr/lib/python3.7/site-packages/ instead of
/usr/local/lib/python3.7/dist-packages/, where Python can't find it.

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Fixes #1269.

[DMRobertson]

Thank you so much for digging into this. It looks like a Herculean effort to me!

Should we be using poetry 1.1.14 here, as in matrix-org/synapse#13285? Perhaps not---I don't think we're generating lockfiles, just reading from them?

No objections otherwise.

[squahtx]

To be honest I still have no idea why only msgpack is affected or how the --no-cache-dir option breaks things, only that it does. This fix is very much a workaround.

Should we be using poetry 1.1.14 here, as in matrix-org/synapse#13285? Perhaps not---I don't think we're generating lockfiles, just reading from them?

I suppose we ought to use poetry 1.14, if that's what Synapse recommends. Though that's orthogonal to the build failures and merits another PR. Not that it should make a difference, since we're just reading lockfiles as you say.

[DMRobertson]

Thanks! SGTM. I assume the dendrite failures are a flake or known problem; they can't be caused by this.

[squahtx]

Agreed, let's merge it.