fix(ci): use Docker network hostname for docs runner checkout

The act_runner and gitea containers share the git_default Docker
network. Use http://gitea:3000 (container DNS name) instead of
localhost, which resolves to the runner container itself.

.gitea/workflows/docs.yml

@@ -13,7 +13,7 @@ jobs:
     steps:
       - name: Checkout
         run: |
-          REPO_URL="http://localhost:3000/kindred/create.git"
+          REPO_URL="http://gitea:3000/kindred/create.git"
           if [ -d .git ]; then
             git fetch "$REPO_URL" main
             git checkout -f FETCH_HEAD
@@ -21,6 +21,15 @@ jobs:
             git clone --depth 1 --branch main "$REPO_URL" .
           fi
 
+      - name: Install mdBook
+        run: |
+          if ! command -v mdbook &>/dev/null; then
+            MDBOOK_VERSION="v0.5.2"
+            wget -q -O mdbook.tar.gz "https://github.com/rust-lang/mdBook/releases/download/${MDBOOK_VERSION}/mdbook-${MDBOOK_VERSION}-x86_64-unknown-linux-musl.tar.gz"
+            tar -xzf mdbook.tar.gz -C /usr/local/bin
+            rm mdbook.tar.gz
+          fi
+
       - name: Build mdBook
         run: mdbook build docs/