Address hadolint warnings/info

[chrisdoherty4]

Hado lint has identified a few improvements to the Dockerfile - implement them.

./Dockerfile:9 DL3018 warning: Pin versions in apk add. Instead of `apk add <package>` use `apk add <package>=<version>`
./Dockerfile:9 DL3019 info: Use the `--no-cache` switch to avoid the need to use `--update` and remove `/var/cache/apk/*` when done installing packages
./Dockerfile:11 DL3059 info: Multiple consecutive `RUN` instructions. Consider consolidation.