Lock pnpm global version in Dockerfiles

[lkostrowski]

Context

Dockerfile installs pnpm globally in container without locked version.

Contrary to lock files, here each image build will install newest pnpm version from npm.

This introduces risk of errors:

1. pnpm release may be broken
2. our code may be incompatible

And it's hard to debug in general.

Since it's on Saleor to maintain working images, we need to ensure stable dependencies

Solution

Find stable, tested version of pnpm and lock it's version in Dockerfile