Issue: #1389

The docker-compose.yml files now contain specific digests like this:

services:
  wordpress:
    image: versionpress/wordpress:php7.2-apache@sha256:64b3c10adfef9ae65a140d5547d670d996c5c852b9a840c71a86a5be4f777fdc

Dockerfiles also lock specific versions:

FROM wordpress:cli@sha256:f8600f5df710c0973ed250e16c1286de7f8344cd8ce0dcba3779931010e956d4

This should lead to more predictable versions running on users' computers.

Implementation notes:

• docker-compose config --resolve-image-digests prints out docker-compose with digests resolved.
• Using image:tag@sha is a nice trick from this article (that article is worth reading anyway!).
• (BTW, the article is by Renovate, a tool that we should consider in the future.)