Docker container build failed: /bin/sh: 1: flask: not found

This line here:

RUN \  . /home/ubuntu/.virtualenvs/bin/activate && \  pip install -r requirements.txt

Will only activate your virtualenv for that specific command

Since virtualenv activation really only boils down to a PATH manipulation, you can do that via ENV

This should work:

ENV PATH=/home/ubuntu/.virtualenvs/bin:$PATHRUN pip install -r requirements.txt

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