What's the best practice for building Docker for Python — using requirements file OR installing individually?

I am trying to create a Docker image for my Python/Django application. Which method would be best to follow?

1. Using requirements file

ADD requirements.txt /code/requirements.txt 
RUN pip install -r requirements.txt

OR

2. Installing packages individually

RUN pip install package1 package2 package3 
RUN pip install package4 package5

One good thing I find with second approach is that cache can be re-used. Each new line of pip install will be added when a new version is being made, ie, one release will have at-most one line on pip install packages.

I feel that first approach will invalidate the cache as requirements file will change when new packages are added, and all the packages are re-installed.

Which method do you prefer?

Please support your answers with reasons.