Update Dockerfile and environment configuration for Gitea runner and Flutter CI integration

images/flutter-ci/Dockerfile

@@ -0,0 +1,18 @@
+ARG FLUTTER_BASE_IMAGE=ghcr.io/cirruslabs/flutter:stable
+FROM ${FLUTTER_BASE_IMAGE}
+
+USER root
+
+# Flutter-jobb i Gitea Actions behöver ofta även Node.js + Git + Bash.
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+        bash \
+        ca-certificates \
+        curl \
+        git \
+        nodejs \
+        npm \
+        openssh-client \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /workspace