Docker build fails to fetch packages from archive.ubuntu.com inside bash script used in Dockerfile

Trying to build a docker image with the execution of a pre-requisites installation script inside the Dockerfile fails for fetching packages via apt-get from archive.ubuntu.com.

Using the apt-get command inside the Dockerfile works flawless, despite being behind a corporate proxy, which is setup via the ENV command in the Dockerfile. Anyway, executing the apt-get command from a bash-script in a terminal inside the resulting docker container or as "postCreateCommand" in a devcontainer.json of Visual Studio Code does work as expected too. But it won't work in my case for the invocation of a bash script from inside a Dockerfile. It simply will tell:

Starting installation of package iproute2
Reading package lists...
Building dependency tree...
The following additional packages will be installed:
libatm1 libcap2 libcap2-bin libmnl0 libpam-cap libxtables12
Suggested packages:
iproute2-doc
The following NEW packages will be installed:
iproute2 libatm1 libcap2 libcap2-bin libmnl0 libpam-cap libxtables12
0 upgraded, 7 newly installed, 0 to remove and 0 not upgraded.
Need to get 971 kB of archives.
After this operation, 3,287 kB of additional disk space will be used.
Err:1 http://archive.ubuntu.com/ubuntu focal/main amd64 libcap2 amd64 1:2.32-1
Could not resolve 'archive.ubuntu.com'

... more output ...

E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/libc/libcap2/libcap2_2.32-1_amd64.deb  Could not resolve 'archive.ubuntu.com'

... more output ...

Just for example a snippet of the Dockerfile looks like this:

FROM ubuntu:20.04 as builderImage

USER root

ARG HTTP_PROXY_HOST_IP='http://172.17.0.1'
ARG HTTP_PROXY_HOST_PORT='3128'
ARG HTTP_PROXY_HOST_ADDR=$HTTP_PROXY_HOST_IP':'$HTTP_PROXY_HOST_PORT

ENV http_proxy=$HTTP_PROXY_HOST_ADDR
ENV https_proxy=$http_proxy
ENV HTTP_PROXY=$http_proxy
ENV HTTPS_PROXY=$http_proxy
ENV ftp_proxy=$http_proxy
ENV FTP_PROXY=$http_proxy

# it is always helpful sorting packages alpha-numerically to keep the overview ;)
RUN apt-get update && \
    apt-get -y upgrade && \
    apt-get -y install --no-install-recommends apt-utils dialog 2>&1 \
    && \
    apt-get -y install \
        default-jdk \
        git \
        python3 python3-pip

SHELL ["/bin/bash", "-c"]

ADD ./env-setup.sh .
RUN chmod +x env-setup.sh && ./env-setup.sh

CMD ["bash"]

The minimal version of the environment script env-setup.sh, which is supposed to be invoked by the Dockerfile, would look like this:

#!/bin/bash

packageCommand="apt-get";
sudo $packageCommand update;
packageInstallCommand="$packageCommand install";
package="iproute2"
packageInstallCommand+=" -y";
sudo $packageInstallCommand $package;

Of course the usage of variables is down to making use of a list for the packages to be installed and other aspects.

Hopefully that has covered everything essential to the question:

Why is the execution of apt-get working with a RUN and as well running the bash script inside the container after creating, but not from the very same bash script while building the image from a Dockerfile?

I was hoping to find the answer with the help of an extensive web-search, but unfortunately I was only able to find anything but an answer to this case.

As pointed out in the comment section underneath the question: