diff --git a/.gitea/ci-image/Dockerfile b/.gitea/ci-image/Dockerfile
new file mode 100644
index 0000000..22d9d6a
--- /dev/null
+++ b/.gitea/ci-image/Dockerfile
@@ -0,0 +1,21 @@
+# 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
diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index a65286f..655193e 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -9,16 +9,16 @@ on:
 jobs:
   lint:
     runs-on: ubuntu-latest
+    # Custom image (see .gitea/ci-image/Dockerfile): the official
+    # sagemath/sagemath image is amd64-only and this runner is arm64, and
+    # ruff/pytest are baked in here so jobs don't reinstall them every run.
     container:
-      image: sagemath/sagemath:10.4
+      image: gitea.piribauer.ch/julian/sage-ci:latest
 
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Install ruff
-        run: sage --pip install ruff
-
       - name: Preparse .sage files
         # Turns each *.sage file into real Python (*.sage.py) so ruff can
         # parse it, and prepends the `from sage.all import *` that `sage`
@@ -40,14 +40,11 @@ jobs:
     runs-on: ubuntu-latest
     needs: lint
     container:
-      image: sagemath/sagemath:10.4
+      image: gitea.piribauer.ch/julian/sage-ci:latest
 
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Install pytest
-        run: sage --pip install pytest
-
       - name: Run tests
         run: sage --python -m pytest tests/