Run `npm install -g corepack@latest` before running `corepack enable` to work around signatures bug

[rmunn]

In sillsdev/web-languageforge#1842 I discovered that the issue where recent versions of PNPM wouldn't install (#1430) was caused by a corepack bug, which is present in the versions of corepack distributed with all current Node versions (corepack 0.30.0) but fixed in corepack 0.31.0 or later. https://www.github.com/nodejs/corepack/issues/612 recommends running npm install -g corepack@latest before running corepack enable, and I found that that works. We should update our Dockerfiles to include that step.