:clipboard: Dockerfile: Clean apt cache, simplify install step for hugo

[jwflory]

This commit makes the following changes:

• apt: Clean the apt cache after installing packages to contribute to a more lean image size.
• Hugo installation: Reduce four commands to two. Makes for easier reading and simpler script logic.

Built locally and verified that hugo binary works as expected:

$ podman run --rm -it localhost/hugo-simp:latest bash
circleci@895338fceafa:~/project$ hugo version
hugo v0.89.0-ADE966B8+extended linux/amd64 BuildDate=2021-11-02T10:00:18Z VendorInfo=gohugoio