To solve the problem of:

E: Unable to locate package python-pip

Run the package update index cmd:

sudo apt update

If not that, then python-pip-whl (which is also a package installer) is available in the universe repository, make sure that's installed and then run:

sudo apt-get install python-pip-whl

Since Python 2 is past its end-of-life, few packages for Python2 are included in 20.04. You have to install pip for Python 2 manually:

First, install Python 2:

sudo apt install python2

Then, follow https://pip.pypa.io/en/stable/installing/ , using python2:

curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
python2 get-pip.py

You can run the second step with sudo. If you don't use sudo, you'll need to change PATH, as suggested by the installation message. Alternatively, and possibly better (since it doesn't change PATH), use

python2 -m pip

whenever you need pip2.

I've found that creating a virtualenv for Python 2.7 installs also pip

$ virtualenv -p python2 venv
$ . venv/bin/activate
$ pip --version
pip 20.0.2 from /home/.../venv/lib/python2.7/site-packages/pip (python 2.7)

In my case, the curl command for downloading get-pip.py gave a syntax error on running sudo python get-pip.py.

But manual download by visiting https://bootstrap.pypa.io/ and downloading get-pip.py worked fine for me.

Pip for Python 2 is not included in the Ubuntu 20.04 repositories.
You need to install pip for Python 2 using the get-pip.py script.

1. Start by enabling the universe repository:

sudo add-apt-repository universe

2. Update the packages index and install Python 2:

sudo apt update 
sudo apt install python2

3. Use curl to download the get-pip.py script:

curl https://bootstrap.pypa.io/get-pip.py --output get-pip.py

4. Once the repository is enabled, run the script as sudo user with python2 to install pip :

sudo python2 get-pip.py

If an error occurs, as a fallback, the specific 2.7 version of get-pip.py can be used:

curl https://bootstrap.pypa.io/pip/2.7/get-pip.py --output get-pip.py

Pip will be installed globally. If you want to install it only for your user, run the command without sudo. The script will also install setuptools and wheel, which allow you to install source distributions

Verify the installation by printing the pip version number:

pip2 --version

The output will look something like this:

 pip 20.0.2 from /usr/local/lib/python2.7/dist-packages/pip (python 2.7)

I specifically needed a Dockerfile file and this is what I have put inside so that it works without errors, I hope it will help someone.

This is Dockerfile file:

FROM ubuntu:latest
RUN apt-get update -y
RUN apt-get install -y python3 python3-dev
WORKDIR /app
COPY .  /app
ENV DEBUG=True
EXPOSE 80

Put python3 instead ${PYPKG} in line 202, and instead python-pip in line 596 in file install.sh of mininet-wifi.