installing python packages in a docker file ?

i have a sample docker file , that i want to use build containers for my data science project. but have few questions around this . 

1. why do we need to install software packages , including gcc compilers .e.t.c . what is included in alpine base image , and do we need all these software packages on top of alpine to install python? 

2. all the python packages needed , like numpy, matplotlib and pandas are included at once, should i try to create layer images . so that , each python package is a layer , instead of all packages built once. what would be a correct way to do this . 

3. in the last statement below , i see install  $PYTHON_PACKAGES  before the software packages ( see below) , what is the order that these get executed , I would think the SOFTWARE_PACKAGES   needed to be installed before the python?

```

    && pip install --no-cache-dir $PYTHON_PACKAGES \

  ...
    && apk add --no-cache --virtual build-dependencies $SOFTWARE_PACKAGES \

```

```

FROM alpine:latest

WORKDIR /var/www/

# SOFTWARE PACKAGES
#   * musl: standard C library
#   * lib6-compat: compatibility libraries for glibc
#   * linux-headers: commonly needed, and an unusual package name from Alpine.
#   * build-base: used so we include the basic development packages (gcc)
#   * bash: -- /bin/bash
#   * git: to ease up clones of repos
#   * ca-certificates: for SSL verification during Pip and easy_install
#   * libgfortran: Fortran shared library
#   * libgcc: contains shared code that would be inefficient to duplicate every time as well as auxiliary helper routines and runtime support
#   * libstdc++: The GNU Standard C++ Library.
#   * openblas: open source implementation of the BLAS(Basic Linear Algebra Subprograms) API with many hand-crafted optimizations for specific processor types
#   * tcl: scripting language
#   * tk: GUI toolkit for the Tcl scripting language
#   * libssl1.0: SSL shared libraries

ENV PACKAGES="\
    dumb-init \
    musl \
    libc6-compat \
    linux-headers \
    build-base \
    bash \
    git \
    ca-certificates \
    libgfortran \
    libgcc \
    libstdc++ \
    openblas \
    tcl \
    tk \
    libssl1.0 \
    "

# PYTHON DATA SCIENCE PACKAGES
ENV PYTHON_PACKAGES="\
    numpy \
    matplotlib \
    pandas \
    "

RUN apk add --no-cache --virtual build-dependencies python3 \
    && apk add --virtual build-runtime \
    build-base python3-dev openblas-dev freetype-dev pkgconfig gfortran \
    && ln -s /usr/include/locale.h /usr/include/xlocale.h \
    && python3 -m ensurepip \
    && rm -r /usr/lib/python*/ensurepip \
    && pip3 install --upgrade pip setuptools \
    && ln -sf /usr/bin/python3 /usr/bin/python \
    && ln -sf pip3 /usr/bin/pip \
    && rm -r /root/.cache \
    && pip install --no-cache-dir $PYTHON_PACKAGES \
    && apk del build-runtime \
    && apk add --no-cache --virtual build-dependencies $SOFTWARE_PACKAGES   \
    && rm -rf /var/cache/apk/*

CMD ["python3"]

```