Use Docker base images to distroless or scratch

## Background
- This repository uses a lot of `alpine`-based Docker base images (see [relevant search](https://github.com/search?l=&q=alpine+path%3A**%2FDockerfile+repo%3AGoogleCloudPlatform%2Fmicroservices-demo&type=code)).
- In light of what happened with Log4j in December 2021, we want to try to shrink the size of Docker base images being used:
- See Google-internal bug, [b/215516492](https://b.corp.google.com/issues/215516492).
- Smaller base images = less vulnerabilities.
- Relevant work from the past:
- #49
- #45

## Task
For each Dockerfile in this repository that uses a distro-ed base image (e.g., `alpine`):
1. Explore how we could remove the need for a distro.
2. If possible, remove it.

Start by inventorying every Dockerfile in the repository and identifying the Alpine or other distro-based base images they use. For each one, investigate whether the service can run without a distro, then verify that any replacements build and the repository's relevant deployment checks still pass.

Written by the indexing model from the issue text.