Docker comment (#4375)

* update comment

* add comment about vcpkg

* dsfs

sdk/core/azure-core/test/libcurl-stress-test/Dockerfile

@@ -16,6 +16,13 @@ RUN ln -s /opt/vcpkg/vcpkg /usr/local/bin/vcpkg
 
 ADD . /src
 WORKDIR /build
+# During CMake generate step VCPKG runs in manifest mode, as such it will sync the packages to the level 
+# of the hash specified in src/azure-sdk-for-cpp/cmake-modules/AzureVcpkg.cmake in the VCPKG_COMMIT_STRING
+# environment variable thus the packages we run with are not the latest versions but the ones the code
+# was developed against. If the builtin-baseline is specified in the vcpkg file then that is the top most
+# version of the packages that will be fetched.
+# So when building from root we need to match the two values. When not building from root if the vcpkg file
+# does not specify a baseline the value set in the cmake file will ensure that we are at the desired level.
 RUN cmake -DCMAKE_BUILD_TYPE=Release -DBUILD_TESTING=ON -DBUILD_TRANSPORT_CURL=ON /src
 RUN cmake --build . --target azure-core-libcurl-stress-test