I ran into this today. It does look like the GPG key expired on the 17th. The devs seem to have updated the key on their pgp site but not in the Docker image. I was able to get around the issue by adding the following lines to a Dockerfile based on the 4.2 image:

RUN mv /etc/apt/sources.list.d/mongodb-org.list /tmp/mongodb-org.list && \
    apt-get update && \
    apt-get install -y curl && \
    curl -o /etc/apt/keyrings/mongodb.gpg https://pgp.mongodb.com/server-4.2.pub && \
    mv /tmp/mongodb-org.list /etc/apt/sources.list.d/mongodb-org.list;
RUN apt-get update