chore: fix an "it was late"

Author: rjaegers

.devcontainer/cpp/Dockerfile

@@ -1,3 +1,5 @@
+# syntax=docker/dockerfile:1
+
 FROM scratch AS downloader-amd64
 ADD --checksum=sha256:53dc06dc3f775282ddff93b2d0974bbe6480761018c61e7a797342a9c7f14e23 \
  https://download.docker.com/linux/static/stable/x86_64/docker-28.2.2.tgz /docker.tgz
@@ -57,8 +59,8 @@ ENV CPM_SOURCE_CACHE=/cache/.cpm-cache
 # hadolint ignore=SC1091
 RUN --mount=type=bind,source=.devcontainer/cpp/apt-requirements-clang.json,target=/tmp/apt-requirements-clang.json \
     --mount=from=downloader,target=/keys \
-    echo /keys/llvm.gpg.key | gpg --dearmor -o /usr/share/keyrings/llvm-snapshot-keyring.gpg \
- && echo /keys/mull.gpg.key | gpg --dearmor -o /usr/share/keyrings/mull-project-mull-stable-archive-keyring.gpg \
+    cat /keys/llvm.gpg.key | gpg --dearmor -o /usr/share/keyrings/llvm-snapshot-keyring.gpg \
+ && cat /keys/mull.gpg.key | gpg --dearmor -o /usr/share/keyrings/mull-project-mull-stable-archive-keyring.gpg \
  && UBUNTU_CODENAME=$(. /etc/os-release; echo "${UBUNTU_CODENAME/*, /}") \
  && echo "deb [signed-by=/usr/share/keyrings/llvm-snapshot-keyring.gpg] http://apt.llvm.org/${UBUNTU_CODENAME}/ llvm-toolchain-${UBUNTU_CODENAME}-${CLANG_VERSION} main" | tee /etc/apt/sources.list.d/llvm.list > /dev/null \
  && echo "deb [signed-by=/usr/share/keyrings/mull-project-mull-stable-archive-keyring.gpg] https://dl.cloudsmith.io/public/mull-project/mull-stable/deb/ubuntu ${UBUNTU_CODENAME} main" | tee /etc/apt/sources.list.d/mull-project-mull-stable.list > /dev/null \