Build fail maybe because of firewall (add-apt-repository)

In my company environment, we have to use proxy.

So I had to modify the ubuntu_circuit_training file like this.

~~~
APT_COMMAND="apt-get -o Acquire::Retries=10 -o Acquire::http::Proxy="http://companyproxyip:8080" -o Acquire::https::Pr
oxy="http://companyproxyip:8080" -y"
~~~

It made me to run this line successfully.

~~~
# Installs basics including add-apt.
RUN ${APT_COMMAND} update && ${APT_COMMAND} install -y --no-install-recommends \
software-properties-common \
curl \
tmux \
vim \
less
~~~

But I can't run this repository stuff.
~~~
# Adds repository to pull versions of python from.
RUN add-apt-repository ppa:deadsnakes/ppa
~~~

I met this error message
~~~
> [ 4/17] RUN add-apt-repository ppa:deadsnakes/ppa:
2.855 Cannot add PPA: 'ppa:~deadsnakes/ubuntu/ppa'.
2.855 ERROR: '~deadsnakes' user or team does not exist.
~~~

I figured out that was related with proxy. I tried several way like as ENV command(http_proxy variable) or COPY command(/etc/apt copy), but not successful till now.
Is there anyone who solved this issue?

I'm running on Ubuntu 20.04 env.

Thank you in advance.

Start with the ubuntu_circuit_training file and reproduce the Docker build at the add-apt-repository ppa:deadsnakes/ppa step using the reported Ubuntu 20.04 proxy environment. Done means the repository can be added and the build proceeds successfully when a proxy is required.

Written by the indexing model from the issue text.