Update gatsby-cli to v4

[quantumfusion]

The latests version of gatsby-cli dropped support for NodeJS12, which is what we have specified in our Dockerfile. Before updating to a more recent version of the cli we need to update the docker image to a newer version as well as update the package.json, package-lock.json as well as address any relevant recommendations from their upgrade guide: https://www.gatsbyjs.com/docs/reference/release-notes/migrating-from-v3-to-v4/