pip command is missing in Dockerfile

When I tried to build the docker container by running this command:

./docker/bin/build_container.sh

I got an error at line:

`RUN pip install -r requirements.txt -I`

because the pip command is not found.

What I fixed on my local is installing python3-pip before running the pip command by adding:

```
USER root
RUN set -xe \
&& apt-get update \
&& apt-get install -y python3-pip
```

into the Dockerfile