diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index 815f95d..96e6966 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -19,6 +19,35 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      # setup-php installs PHP 8.3+ through php-builder, which apt-installs ~70
+      # -dev packages before unpacking the build (#178). Ubuntu's own image
+      # deletes the .debs after install, so every job re-downloads them from
+      # the archive. Keep them and restore them from the cache server, which
+      # lives in the cluster, turning a WAN download into a local one.
+      #
+      # The dep list is the same for every PHP version on the builder path, so
+      # one shared key serves them all. Only the .debs are cached, never
+      # /var/lib/apt/lists — a stale index is how you get 404s on install.
+      - name: Keep downloaded .debs
+        run: |
+          sudo rm -f /etc/apt/apt.conf.d/docker-clean
+          echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' \
+            | sudo tee /etc/apt/apt.conf.d/99keep-downloaded-packages >/dev/null
+
+      - name: Cache apt packages
+        uses: actions/cache@v3
+        with:
+          path: /var/cache/apt/archives/*.deb
+          key: apt-php-builder-deps-${{ runner.arch }}-v1
+
+      # Temporary, for #178: shows whether the cache actually served the
+      # builder's dependencies, and times the step that consumes them.
+      - name: Report restored .debs
+        run: |
+          echo "restored .debs: $(ls /var/cache/apt/archives/*.deb 2>/dev/null | wc -l)"
+          echo "restored size:  $(du -sh /var/cache/apt/archives 2>/dev/null | cut -f1)"
+          date -u +'setup-php start: %H:%M:%S'
+
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
@@ -54,6 +83,35 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      # setup-php installs PHP 8.3+ through php-builder, which apt-installs ~70
+      # -dev packages before unpacking the build (#178). Ubuntu's own image
+      # deletes the .debs after install, so every job re-downloads them from
+      # the archive. Keep them and restore them from the cache server, which
+      # lives in the cluster, turning a WAN download into a local one.
+      #
+      # The dep list is the same for every PHP version on the builder path, so
+      # one shared key serves them all. Only the .debs are cached, never
+      # /var/lib/apt/lists — a stale index is how you get 404s on install.
+      - name: Keep downloaded .debs
+        run: |
+          sudo rm -f /etc/apt/apt.conf.d/docker-clean
+          echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' \
+            | sudo tee /etc/apt/apt.conf.d/99keep-downloaded-packages >/dev/null
+
+      - name: Cache apt packages
+        uses: actions/cache@v3
+        with:
+          path: /var/cache/apt/archives/*.deb
+          key: apt-php-builder-deps-${{ runner.arch }}-v1
+
+      # Temporary, for #178: shows whether the cache actually served the
+      # builder's dependencies, and times the step that consumes them.
+      - name: Report restored .debs
+        run: |
+          echo "restored .debs: $(ls /var/cache/apt/archives/*.deb 2>/dev/null | wc -l)"
+          echo "restored size:  $(du -sh /var/cache/apt/archives 2>/dev/null | cut -f1)"
+          date -u +'setup-php start: %H:%M:%S'
+
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
@@ -97,6 +155,35 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      # setup-php installs PHP 8.3+ through php-builder, which apt-installs ~70
+      # -dev packages before unpacking the build (#178). Ubuntu's own image
+      # deletes the .debs after install, so every job re-downloads them from
+      # the archive. Keep them and restore them from the cache server, which
+      # lives in the cluster, turning a WAN download into a local one.
+      #
+      # The dep list is the same for every PHP version on the builder path, so
+      # one shared key serves them all. Only the .debs are cached, never
+      # /var/lib/apt/lists — a stale index is how you get 404s on install.
+      - name: Keep downloaded .debs
+        run: |
+          sudo rm -f /etc/apt/apt.conf.d/docker-clean
+          echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' \
+            | sudo tee /etc/apt/apt.conf.d/99keep-downloaded-packages >/dev/null
+
+      - name: Cache apt packages
+        uses: actions/cache@v3
+        with:
+          path: /var/cache/apt/archives/*.deb
+          key: apt-php-builder-deps-${{ runner.arch }}-v1
+
+      # Temporary, for #178: shows whether the cache actually served the
+      # builder's dependencies, and times the step that consumes them.
+      - name: Report restored .debs
+        run: |
+          echo "restored .debs: $(ls /var/cache/apt/archives/*.deb 2>/dev/null | wc -l)"
+          echo "restored size:  $(du -sh /var/cache/apt/archives 2>/dev/null | cut -f1)"
+          date -u +'setup-php start: %H:%M:%S'
+
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with: