[Dockerfile] import-meta-env - Update target node version to 22

[meissadia]

https://github.com/cfpb/sbl-frontend/blob/main/Dockerfile#L8-L10

RUN npx pkg ./node_modules/@import-meta-env/cli/bin/import-meta-env.js \
  -t node18-alpine-x64 \
  -o import-meta-env-alpine

The latest version of Node supported by pkg is v18
https://www.npmjs.com/package/pkg-fetch#binary-compatibility

The source repo for pkg is archived as of the start of 2024, so no further updates expected there.
https://github.com/vercel/pkg

The current target (node18) works fine despite upgrading the underlying node version to 22 but, to ensure future compatibility, we should explore alternatives.

Questions:

• Is there an alternative to pkg we can use?
• Is there a port of the pkg repo that supports newer Node versions?