fix(content): align compose demo Dockerfiles to COPY over ADD

The app and django compose demos used ADD for plain local files, but the
book's in-text versions of the same examples (11.3_usage.md, 11.6_django.md)
and its own guidance (7.2/7.3, appendix best_practices) all use COPY. Align
the demos with the documented practice.

11_compose/demo/django/Dockerfile

@@ -2,6 +2,6 @@ FROM python:3
 ENV PYTHONUNBUFFERED 1
 RUN mkdir /code
 WORKDIR /code
-ADD requirements.txt /code/
+COPY requirements.txt /code/
 RUN pip install -r requirements.txt
-ADD . /code/
+COPY . /code/