Installing clang ubuntu docker

Beginners
1

Hi folks, I’m trying to create a docker image with clang 13 to compile CPython, but I keep getting the following error

Step 6/9 : RUN add-apt-repository "http://apt.llvm.org/focal/ llvm-toolchain-focal-13 main"
 ---> Running in 401353b306da
Hit:2 http://security.ubuntu.com/ubuntu focal-security InRelease
Hit:3 http://archive.ubuntu.com/ubuntu focal InRelease
Hit:4 http://archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:5 http://archive.ubuntu.com/ubuntu focal-backports InRelease
Ign:1 https://apt.llvm.org/focal focal InRelease
Err:6 https://apt.llvm.org/focal focal Release
  404  Not Found [IP: 151.101.206.49 443]
Reading package lists...
E: The repository 'http://apt.llvm.org/focal focal Release' does not have a Release file.
The command '/bin/sh -c add-apt-repository "http://apt.llvm.org/focal/ llvm-toolchain-focal-13 main"' returned a non-zero code: 100

Here’s my dockerfile

FROM ubuntu:focal

WORKDIR /opt/cpython-3.9

RUN apt-get update
RUN apt-get install --yes wget gnupg2 software-properties-common

RUN wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key| apt-key add -
RUN add-apt-repository "http://apt.llvm.org/focal/ llvm-toolchain-focal-13 main"

RUN apt-get update && apt-get upgrade
RUN apt-get install --yes clang-13 lld-13 libncurses-dev libgdbm-dev libz-dev tk-dev libsqlite3-dev libreadline-dev liblzma-dev libffi-dev libssl-dev

RUN apt-get update -qq \
    && apt-get install --yes build-essential curl \
    curl -fsSL https://www.python.org/ftp/python/3.9.6/Python-3.9.6.tgz \
    | tar xz --strip-components 1 \
    && CC=clang ./configure --with-lto=full --enable-shared --enable-optimizations --prefix /usr/local/cpython-3.9 \
    && make \
    && make install

Am I missing something? Thanks in advance!