feat: add REPO/BRANCH build args for llama.cpp (#59)

- Introduce ARG REPO and ARG BRANCH to replace the hardcoded git clone with: `git clone -b ${BRANCH} --single-branch --recursive ${REPO}` . This allows overriding the llama.cpp repository and branch at build time via `--build-arg`.

- Update `docs/building.md` to recommend using `--build-arg` instead of updating the file

toolboxes/Dockerfile.rocm7-nightlies

@@ -53,7 +53,9 @@ RUN printf '%s\n' \
   && echo 'source /etc/profile.d/rocm.sh' >> /etc/bashrc
 
 WORKDIR /opt/llama.cpp
-RUN git clone --recursive https://github.com/ggerganov/llama.cpp.git . \
+ARG REPO=https://github.com/ggerganov/llama.cpp.git
+ARG BRANCH=master
+RUN git clone -b ${BRANCH} --single-branch --recursive ${REPO} . \
   && git clean -xdf \
   && git submodule update --recursive