Add support for pre-built docker image (update only app layer)

[glensc]

Problem is that --cache-from has no effect in GitHub Actions builds. The builds are always full, compiling pecl/mongodb from source:

Example:

• 1m 39s: https://github.com/perftools/xhgui/runs/1610535620

Let's save (some) green energy and don't build the extension from the source on each pull request. This results in a bit bigger image, as the source is added twice to layers, but the image is still accurate.

Example:

• 17s: https://github.com/perftools/xhgui/runs/1610589204