DEVX-68: feat: add pre-built Docker runner images and tested image build/push tools

docker/ci-base/Dockerfile

@@ -0,0 +1,26 @@
+# ci-base — lightweight image for CI jobs that only need devx core + tea.
+#
+# Used by: detect-type, detect-changes, validate-commit-msg, pr-review,
+#          auto-merge, sync-wiki, vikunja, configure-repo, discover-runners,
+#          molecule-report, discover-integration-runners
+#
+# Jobs using this image: setup is instant (ln -s /opt/venv .venv)
+# No pip install needed — devx and all deps are pre-installed.
+
+FROM gitea/runner-images:ubuntu-latest
+
+# Create a virtual environment with all deps pre-installed
+RUN python3 -m venv /opt/venv
+ENV PATH="/opt/venv/bin:/root/.local/bin:$PATH"
+
+# Install devx from local source (build context = devx repo root)
+COPY . /tmp/devx
+RUN pip install --no-cache-dir --upgrade pip setuptools wheel \
+    && pip install --no-cache-dir /tmp/devx[ci] \
+    && rm -rf /tmp/devx
+
+# Install tea CLI (for Gitea API operations in CI)
+RUN python3 -m devx.tools.install_tools --tool tea
+
+# Workspace directory (actions/checkout mounts repo here)
+WORKDIR /workspace