Build and push pre-built Docker images periodically

[robdimsdale]

In #627 we switched to using pre-built Docker images for some of the actions that are invoked most frequently. This reduced the rate of GitHub actions encountering errors during these builds - as pulling images is more reliable than building them.

However, a consequence of that change is that these Docker images are no longer getting built every time the build runs. Therefore they have the potential to go stale.

As detailed in #627, stale images isn't a significant risk because the images themselves aren't a particularly high security concern. However, we should update them periodically. An initial proposal could be doing this once per week. However, we could build and push them more frequently (e.g. daily) if we prefer.

[robdimsdale]

@brayanhenao and I just made a change to the release/find-asset action and were very confused to not see our changes reflected in the running job until we realized that the image was pre-built and hence our updates weren't going to be reflected until we rebuilt the image.

It was a little confusing to find the correct set of credentials to create and push the image, and I would rather encapsulate this in automation rather than documentation.

So, in addition to regularly rebuilding images for security/keeping up to date with libraries/dependencies, we should also trigger a rebuild when the action files themselves change.