Create Docker image build action

.github/workflows/docker.yml

@@ -0,0 +1,24 @@
+name: Build Docker image
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build the Docker image
+        run: docker build . --tag mitmproxy-pretendo
+
+      - name: Save the Docker image
+        run: docker save mitmproxy-pretendo -o mitmproxy-pretendo.tar
+
+      - name: Upload the Docker image
+        uses: actions/upload-artifact@v4
+        with:
+          name: mitmproxy-pretendo-docker-image-${{ github.sha }}
+          path: mitmproxy-pretendo.tar