Update build process (Maven plugin -> Dockerfile)

The frontend is currently built using a Maven plugin. We did this because the application was shipped as an executable `.jar` file. Now that we're adopting Docker, we should build the frontend as part of the Docker image, just like we do the frontend. This would essentially be copy/pasting the build steps from the Maven plugin to the Dockerfile.

One caveat will be copying the built frontend properly so Spring Boot can load it.

Locate the existing Maven frontend build plugin steps and inspect the Dockerfile, then determine how the built frontend is copied for Spring Boot to load. Done means the frontend builds as part of the Docker image and is served correctly by the application.

Written by the indexing model from the issue text.