[CI] Fix previews

Summary:
Some docker files are located at the project root so they can use the in-tree version of their dependencies. But the previews build rules have not been updated accordingly, and the context can currently only be a subdirectory of the root.
This diff fixes this issue by making the context be the project root by default, and updating the builds.

Test Plan:

./contrib/teamcity/build-configurations.py preview-cashtab
./contrib/teamcity/build-configurations.py preview-chronik.e.cash

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17162