chore: get rid of shellcheck ignore

Don't source /etc/os-release but grep the value we want from it

Author: rjaegers

.devcontainer/cpp/Dockerfile

@@ -38,13 +38,12 @@ ENV CCACHE_DIR=/cache/.ccache \
     CPM_SOURCE_CACHE=/cache/.cpm-cache
 
 # Install clang toolchain and mull mutation testing framework
-# hadolint ignore=SC1091
 RUN --mount=type=bind,source=.devcontainer/cpp/apt-requirements-clang.json,target=/tmp/apt-requirements-clang.json \
     --mount=type=cache,target=/var/cache/apt,sharing=locked \
     --mount=type=cache,target=/var/lib/apt,sharing=locked \
  wget -qO - https://apt.llvm.org/llvm-snapshot.gpg.key | gpg --dearmor -o /usr/share/keyrings/llvm-snapshot-keyring.gpg \
  && wget -qO - https://dl.cloudsmith.io/public/mull-project/mull-stable/gpg.41DB35380DE6BD6F.key | gpg --dearmor -o /usr/share/keyrings/mull-project-mull-stable-archive-keyring.gpg \
- && UBUNTU_CODENAME=$(. /etc/os-release; echo "${UBUNTU_CODENAME/*, /}") \
+ && UBUNTU_CODENAME=$(grep '^UBUNTU_CODENAME=' /etc/os-release | cut -d= -f2) \
  && echo "deb [signed-by=/usr/share/keyrings/llvm-snapshot-keyring.gpg] http://apt.llvm.org/${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 \
  && echo -e 'Package: *\nPin: origin "apt.llvm.org"\nPin-Priority: 1000' > /etc/apt/preferences \