Docker: Allow optional building of the frontend

[jippi]

For Pixelfed users/forks that uses a modified frontend, this will allow them to use the new Docker setup out of the box.

With DOCKER_APP_BUILD_FRONTEND=0 (default)

By default no building will happen, just a couple of minor "wasteful" Docker build steps that are effectively NOOPs.

With DOCKER_APP_BUILD_FRONTEND=1

npm install --dev are installed in the intermediate image followed by npm run production that builds the frontend.

The new/updated files are then copied into the final container image, overriding the default pre-built assets.

Updating

• Once merged, pull the change into your fork/repo
• Run ./docker/dottie sync to update your existing .env file with the new one
• Run ./docker/dottie set DOCKER_APP_BUILD_FRONTEND=1 to enable the new setting
• Run docker compose build to kick off the build process
• Run docker compose up -d to recreate your containers with the new custom frontend

[dansup]

@jippi Hey, can you target the staging branch?