Optimized image build time

Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1.7
 # Gitea Actions CI image for C/C++ + Vulkan projects.
 # This extends the Ubuntu image commonly used by Gitea/act_runner so JS actions
 # such as actions/checkout keep working.
@@ -24,7 +25,8 @@ ENV STRIP=llvm-strip
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 
 # Base compiler/build/Python/Vulkan packages from Ubuntu.
-RUN set -eux; \
+RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
+    set -eux; \
     apt-get update; \
     apt-get install -y --no-install-recommends \
       ca-certificates \
@@ -55,7 +57,8 @@ RUN set -eux; \
     rm -rf /var/lib/apt/lists/*
 
 # LLVM/Clang from apt.llvm.org.
-RUN set -eux; \
+RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
+    set -eux; \
     codename="$(. /etc/os-release && echo "${VERSION_CODENAME}")"; \
     curl -fsSL https://apt.llvm.org/llvm-snapshot.gpg.key \
       | gpg --dearmor -o /usr/share/keyrings/llvm-snapshot.gpg; \
@@ -76,7 +79,8 @@ RUN set -eux; \
 
 # Optional LunarG Vulkan SDK repo. Disabled by default because Ubuntu distro
 # Vulkan packages are more stable for CI and usually enough for building/testing.
-RUN set -eux; \
+RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
+    set -eux; \
     if [ "${USE_LUNARG_VULKAN}" = "true" ]; then \
       codename="$(. /etc/os-release && echo "${VERSION_CODENAME}")"; \
       curl -fsSL https://packages.lunarg.com/lunarg-signing-key-pub.asc \
@@ -101,7 +105,8 @@ RUN set -eux; \
     update-alternatives --install /usr/bin/llvm-strip llvm-strip "/usr/bin/llvm-strip-${LLVM_VERSION}" 100
 
 # Meson in an isolated venv avoids Debian/Ubuntu PEP 668 externally-managed pip errors.
-RUN set -eux; \
+RUN --mount=type=cache,target=/root/.cache/pip \
+    set -eux; \
     python3 -m venv /opt/meson-venv; \
     /opt/meson-venv/bin/python -m pip install --upgrade pip setuptools wheel; \
     if [ "${MESON_VERSION}" = "latest" ]; then \
@@ -117,7 +122,6 @@ RUN set -eux; \
     clang --version; \
     clang++ --version; \
     meson --version; \
-    ninja --version; \
-    vulkaninfo --summary || true
+    ninja --version
 
 CMD ["bash"]