Which base image to use?

[c0c0n3]

Is your feature request related to a problem? Please describe.

Is Alpine still the best Docker base image to use for releasing QL? It definitely seems to be in terms of size I think, but PR #612 highlighted it may not necessarily be the best when it comes to performance or security---follow the links posted in the PR comments to read some articles about that.

Describe the solution you'd like

Do some digging to find out what base image to use:

1. to build the official QL images released on DockerHub;
2. to test in CircleCI;
3. for local testing---i.e. devs testing on their own boxes.

Ideally, it should be the same image across the board, but there's likely to be trade-offs for each case---e.g. Alpine could be optimal for local development b/c of its small size whereas Python slim could be better for e.g. (2). Luckily, PR #612 introduced the machinery to enable us to use different base images in different situations, so we can piggyback on that if we realise we'd like to use different base images for (1), (2) and (3).

Describe alternatives you've considered

Do nothing. Thanks to PR #612, if our end users aren't happy with our choice of base image, they can easily rebuild their own QL image from a base image of their choice.

Additional context

See also #592 about runtime performance.