- Add missing build dependencies for `sharp` node module (dependency of
`gatsby-plugin-sharp`).
- Add ARG instructions for firebase variables, allowing them to be set
via --build-arg flags when building the image.
- Use `npm ci` instead of `npm install`, and execute it after copying
only package*.json. This prevents having to fetch all dependencies if
package*.json hasn't changed between builds.
- Use `node:alpine` instead of `node` - not an improvement per-se,
but I happen to be more familiar with adding build dependecies in the
alpine based node image.
