Can't extend docker image

**Use case**

I want to extend the scikit learn prebuilt docker file (https://github.com/aws/sagemaker-scikit-learn-container/blob/master/docker/0.23-1/base/Dockerfile.cpu)and add some of my own stuff needed for the inference pipeline. This includes additional code besides the inference script itself and some ENV settings to point out where things can be found (SAGEMAKER_PROGRAM and SAGEMAKER_SUBMIT_DIRECTORY)

In the original dockerfile in:
- line 40 the kitware PPA is added.
- lines 46 and 47 kitware packages are installed
- lines 59 and 60 the GPG key of the kitware PPA is removed.

In my extension docker I need to add extra ubuntu package. If I do a:
`apt-get update`
I get an error caused by kitware.
`#6 1.891 Err:9 https://apt.kitware.com/ubuntu bionic Release
#6 1.891 Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate. Could not handshake: Error in the certificate verification. [IP: 66.194.253.25 443]`

**Workaround**

I hacked it by doing this:
`RUN apt-get update -y || apt-get install ssh`

**Expected behavior**

Extending the docker image is possible.