Lock pnpm version in Dockerfile to 7.5.2

[lkostrowski]

In the future this should prevent unexpected behavior, when image is downloading newer version than we tested with

[vercel]

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[krzysztofwolski]

@lkostrowski maybe we should add ignore flag instead of forcing the exact version?
https://pnpm.io/errors#err_pnpm_peer_dep_issues

[lkostrowski]

@lkostrowski maybe we should add ignore flag instead of forcing the exact version? https://pnpm.io/errors#err_pnpm_peer_dep_issues

I don't think this is a good idea.

There are reasons why package managers are showing errors - we should maintain dependencies versions to work correctly.
If we ignore errors, we can face some silent bugs

The exact version will prevent the case from broken Storefront, caused by Storefront using v6, Dockerfile was installing V7.
I'd like to prevent this in the future. Without a locked version, PNPM can install v8 and break it again.

[zaiste]

The suggested way (their docs) to install pnpm is by using the standalone script. This also locks the version, but it also provides a pre-built binary. A nitpick, but it may help to make it more stable.

[typeofweb]

@zaiste they suggest 5 different ways of installing pnpm. What are the benefits of using a script instead of npm here?
BTW, we're using Corepack on Vercel.