Simplify custom InfiniTime dir instructions for docker workflow (#179)

Update the `docker` example for using a custom InfiniTime repo.

We're already mounting the InfiniTime repo into the container.
So it is easier to mount it over the default location instead of mounting it
somewhere else and then provide the "somewhere else" path
with the `InfiniTime_DIR` cmake variable.

Co-authored-by: NeroBurner <pyro4hell@gmail.com>

Author: coffandro

README.md

@@ -131,9 +131,9 @@ podman run --rm -it -v ${PWD}:/sources infinisim-build
 ```
 
 By default this builds the simulator using the InfiniTime files from the submodule in your `${PWD}`.
-If you want to use a different repository, you got to mount it and pass the path to the `INFINITIME_DIR` variable:
+If you want to use a different repository, the easiest way is to mount over the default submodule location in the container at `/sources/InfiniTime`:
 ```sh
-docker run --rm -it -v ${PWD}:/sources -v ${PWD}/../InfiniTime:/infinitime -e INFINITIME_DIR=/infinitime  --user $(id -u):$(id -g) infinisim-build
+docker run --rm -it -v ${PWD}:/sources -v ${PWD}/../InfiniTime:/sources/InfiniTime --user $(id -u):$(id -g) infinisim-build
 ```
 
 Other CMake generation and build arguments can be passed to the `GENERATE_ARGS` and `BUILD_ARGS` variables: