Improve Dockerfile with Hadolint suggestions

[channelbeta]

This PR updates the Dockerfile according to Hadolint suggestions.

The ignored checks (3013 and 3018) are related to pinning package versions.

Before:

docker run --rm -i hadolint/hadolint < Dockerfile
-:3 DL3013 warning: Pin versions in pip. Instead of `pip install <package>` use `pip install <package>==<version>` or `pip install --requirement <requirements file>`
-:3 DL3042 warning: Avoid use of cache directory with pip. Use `pip install --no-cache-dir <package>`
-:5 DL3059 info: Multiple consecutive `RUN` instructions. Consider consolidation.
-:5 DL3018 warning: Pin versions in apk add. Instead of `apk add <package>` use `apk add <package>=<version>`
-:5 DL3019 info: Use the `--no-cache` switch to avoid the need to use `--update` and remove `/var/cache/apk/*` when done installing packages

After:

docker run --rm -i hadolint/hadolint < Dockerfile

...no complaints :D