Speed up Docker builds with BuildKit ccache mount

Enable ccache in the Docker build by:
- Adding ccache to the builder stage's apt packages
- Using a BuildKit cache mount (--mount=type=cache) for /root/.cache/ccache
  so the compiler cache persists across rebuilds
- Setting CMAKE_C/CXX_COMPILER_LAUNCHER=ccache in the cmake invocation
- Adding the BuildKit syntax directive and BUILDKIT_INLINE_CACHE=1 build arg
  in the CI workflow

Also modernise the Dockerfile syntax:
- Use '=' form for ENV directives (required by newer Docker versions)
- Use uppercase AS for multi-stage build aliases

Co-authored-by: Kiro <kiro-agent@users.noreply.github.com>

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -1015,7 +1015,7 @@ jobs:
           sudo apt update
           sudo apt install openjdk-11-jdk-headless
       - name: Build docker image
-        run: docker build -t cbmc .
+        run: docker build --build-arg BUILDKIT_INLINE_CACHE=1 -t cbmc .
       - name: Smoke test goto-cc
         run: docker run -v ${PWD}/.github/workflows/smoke_test_assets:/mnt/smoke -t cbmc goto-cc -o /mnt/smoke/test.goto /mnt/smoke/test.c
       - name: Smoke test cbmc

Dockerfile

@@ -1,6 +1,7 @@
-FROM ubuntu:20.04 as builder
-ENV DEBIAN_FRONTEND noninteractive
-ENV DEBCONF_NONINTERACTIVE_SEEN true
+# syntax=docker/dockerfile:1
+FROM ubuntu:20.04 AS builder
+ENV DEBIAN_FRONTEND=noninteractive
+ENV DEBCONF_NONINTERACTIVE_SEEN=true
 # Timezone data is needed during the installation of dependencies,
 # since cmake depends on the tzdata package. In an interactive terminal,
 # the user selects the timezone at installation time. Since this needs
@@ -18,11 +19,18 @@ RUN echo 'tzdata tzdata/Areas select Etc' | debconf-set-selections; \
     flex \
     bison \
     libxml2-utils \
-    patch
+    patch \
+    ccache
 COPY . /tmp/cbmc
 WORKDIR /tmp/cbmc
-RUN cmake -S . -Bbuild -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=/usr/bin/gcc -DCMAKE_CXX_COMPILER=/usr/bin/g++ && cd build; ninja -j2
+RUN --mount=type=cache,target=/root/.cache/ccache \
+    cmake -S . -Bbuild -G Ninja -DCMAKE_BUILD_TYPE=Release \
+      -DCMAKE_C_COMPILER_LAUNCHER=ccache \
+      -DCMAKE_CXX_COMPILER_LAUNCHER=ccache \
+      -DCMAKE_C_COMPILER=/usr/bin/gcc \
+      -DCMAKE_CXX_COMPILER=/usr/bin/g++ && \
+    cd build && ninja -j2
 
-FROM ubuntu:20.04 as runner
+FROM ubuntu:20.04 AS runner
 COPY --from=builder /tmp/cbmc/build/bin/* /usr/local/bin/
 RUN apt-get update && apt-get install -y gcc