Add dockerfile and ci config

.gitea/workflows/build.yaml

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+name: Build Flutter SDK Docker Image
+on:
+  schedule:
+    - cron: '0 4 * * 0'
+  workflow_run:
+    types:
+      - requested
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+
+### Setup
+
+      - name: Install docker
+        uses: papodaca/install-docker-action@main
+
+      - name: Install dependencies
+        run: apt update && apt install -y curl
+
+      - name: Check out repository code
+        uses: actions/checkout@v3
+      
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Docker Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ vars.DOCKER_REGISTRY_HOST }}
+          username: ${{ secrets.DOCKER_REGISTRY_USERNAME }}
+          password: ${{ secrets.DOCKER_REGISTRY_PASSWORD }}
+
+### Build and push docker image
+
+      - name: Build and push, channel 'stable'
+        run: |
+          export CHANNEL=stable
+          export IMAGE_WITH_TAG=${{ vars.DOCKER_REGISTRY_HOST }}/${{ vars.DOCKER_REPO_AND_IMAGE }}:stable
+          docker buildx build . --build-arg CHANNEL=$CHANNEL -t $IMAGE_WITH_TAG --no-cache --push

Dockerfile

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+FROM ubuntu:22.04
+
+ARG CHANNEL=stable
+
+RUN apt update
+RUN apt install -y curl git wget unzip
+RUN apt clean
+    # download Flutter SDK from Flutter Github repo \
+RUN git clone -b ${CHANNEL} https://github.com/flutter/flutter.git /usr/local/flutter
+
+# Set flutter environment path
+ENV PATH="/usr/local/flutter/bin:/usr/local/flutter/bin/cache/dart-sdk/bin:${PATH}"
+
+RUN flutter doctor
+
+# Propably not needed since we clone that branch above, but to be sure
+RUN flutter channel ${CHANNEL}
+RUN flutter config --enable-web
+
+RUN flutter upgrade
+RUN flutter precache
+RUN flutter --version