feat: cache edx-platform checkout in image build

Similar to what we need with the MFE image, we leverage Docker's ADD
directive to implement caching of the edx-platform git checkout step.

See: overhangio/tutor-mfe#163
https://docs.docker.com/reference/dockerfile/#add

changelog.d/20240607_111921_regis_edx_platform_code_cache.md

@@ -0,0 +1 @@
+- 💥[Improvement] Ensure that the edx-platform repository git checkout is cached by Docker during image build. This means that the cache will automatically be cleared any time there is an upstream change. Thus, it is no longer necessary to run `tutor images build --no-cache` just to fetch the latest edx-platform changes. For this to work, any GitHub repository referenced by `EDX_PLATFORM_REPOSITORY` needs to end with ".git". Make sure that this is the case if you have modified the value of this setting in the past. (by @regisb)

tutor/templates/build/openedx/Dockerfile

@@ -38,9 +38,9 @@ RUN $PYENV_ROOT/versions/$PYTHON_VERSION/bin/python -m venv /openedx/venv
 FROM minimal as code
 ARG EDX_PLATFORM_REPOSITORY={{ EDX_PLATFORM_REPOSITORY }}
 ARG EDX_PLATFORM_VERSION={{ EDX_PLATFORM_VERSION }}
-RUN mkdir -p /openedx/edx-platform && \
-    git clone $EDX_PLATFORM_REPOSITORY --branch $EDX_PLATFORM_VERSION --depth 1 /openedx/edx-platform
+RUN mkdir -p /openedx/edx-platform
 WORKDIR /openedx/edx-platform
+ADD --keep-git-dir=true $EDX_PLATFORM_REPOSITORY#$EDX_PLATFORM_VERSION .
 
 # Identify tutor user to apply patches using git
 RUN git config --global user.email "[email protected]" \