Only build production stage in pipeline

Viaduct uses a multi-stage Dockerfile. Parts of the build process that are shared between development and production, can be reused in the Dockerfile. During the current transition from Webpack to Vite, we have different build stages for both build systems. Currently, the pipeline builds both stages, which makes the pipeline unnecessarily slow. This commit configures GitLab to only build the production stage, which is the only one that will be deployed.