Added automatic docker build on merge to master

This PR includes:
- Automatic build & push to docker hub when pushing to master, or when triggering a workflow dispatch on master
- Automatic docker build on pull requests to validate changes
- Updated the docker-compose.yaml to use the container images, rather than building locally
- Added support for defining backend settings using the container environment 
  -> All options defined in config.toml can now be loaded instead from the environment
  -> Order of precedence is environment definition, config.toml, and finally default inline configuration defined in config.ts
- Added support for defining frontend settings using the container environment
  -> Added a dynamic api route to load the container environment definitions on the server, and provide them to the client
  -> Added a library function to fetch from the newly created API, caching the response in the session storage
  -> Modified existing calls to `process.env` to use the new library function
  -> Left the initial statically compiled environment definitions in place as a backup definition, if no environment definitions are provided

Remaining tasks todo before able to merge to [ItzCrazyKns/Perplexica](https://github.com/ItzCrazyKns/Perplexica):
- Add secret definitions for `DOCKER_USERNAME` and `DOCKER_PASSWORD` to [ItzCrazyKns/Perplexica](https://github.com/ItzCrazyKns/Perplexica) to ensure push to dockerhub works on base branch
- Update documentation with information about changes

.github/workflows/docker-build.yml

@@ -0,0 +1,48 @@
+name: Build and Push Docker images
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: Log in to Docker Hub
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v2
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+
+      - name: Build frontend Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: app.dockerfile
+          push: ${{ (github.event_name != 'pull_request') && (github.ref == 'refs/heads/master') }}
+          tags: ${{ secrets.DOCKER_USERNAME }}/perplexica-frontend:latest
+
+      - name: Build backend Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: backend.dockerfile
+          push: ${{ (github.event_name != 'pull_request') && (github.ref == 'refs/heads/master') }}
+          tags: ${{ secrets.DOCKER_USERNAME }}/perplexica-backend:latest
+
+      - name: Log out from Docker Hub
+        if: github.event_name != 'pull_request'
+        run: docker logout