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Use COPY instead of ADD for files and folders

Problematic code:

FROM python:3.4
ADD requirements.txt /usr/src/app/

Correct code:

FROM python:3.4
COPY requirements.txt /usr/src/app/

Rationale:

https://docs.docker.com/develop/develop-images/instructions/#add-or-copy

For other items (files, directories) that do not require ADD’s tar auto-extraction capability, you should always use COPY.

Rule also implemented in https://github.com/RedCoolBeans/dockerlint/blob/master/src/checks.coffee

Exceptions:

Consequently, the best use for ADD is local tar file auto-extraction into the image.