diff --git a/.gitea/ci-image/Dockerfile b/.gitea/ci-image/Dockerfile
index 22d9d6a..fa9b11c 100644
--- a/.gitea/ci-image/Dockerfile
+++ b/.gitea/ci-image/Dockerfile
@@ -18,4 +18,10 @@ FROM condaforge/miniforge3:latest
 RUN mamba install -y -c conda-forge sage ruff pytest \
     && mamba clean -afy
 
+# actions/checkout@v4 (and other JS-based actions) run via `node` inside this
+# container -- act_runner doesn't inject a runtime of its own when a custom
+# `container:` image is set, so one has to be present here.
+RUN mamba install -y -c conda-forge nodejs \
+    && mamba clean -afy
+
 WORKDIR /workspace