Pipeline setup with Dockerfile

.gitea/ci-image/Dockerfile

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+# Custom CI image for the arm64 (Raspberry Pi) Gitea Actions runner.
+#
+# The official sagemath/sagemath image is amd64-only; conda-forge's `sage`
+# package is the one place SageMath is actually published for linux-aarch64,
+# so this bakes it (plus the lint/test tools) into a single image, built and
+# pushed once rather than reinstalled on every CI run.
+#
+# Build & push (run directly on the Pi, or any arm64 machine with Docker):
+#   docker build -t gitea.piribauer.ch/julian/sage-ci:latest -f .gitea/ci-image/Dockerfile .
+#   docker login gitea.piribauer.ch -u julian
+#   docker push gitea.piribauer.ch/julian/sage-ci:latest
+#
+# Rebuild and re-push whenever this Dockerfile changes (e.g. bumping the sage
+# version) or the pinned tool versions need updating.
+
+FROM condaforge/miniforge3:latest
+
+RUN mamba install -y -c conda-forge sage ruff pytest \
+    && mamba clean -afy
+
+WORKDIR /workspace