[bitcoin-abc][web] Moved explorer dockerfile to root so it can access Cargo.lock

Summary: We move the dockerfile out of bitcoin-abc/web/explorer and into bitcoin-abc so that Cargo.lock is in the docker context. Then changed the build-configurations file to include the explorer.Dockerfile in the preview-explorer build

Test Plan: ninja check-functional @bot preview-explorer

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, tobias_ruck

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, tobias_ruck

Subscribers: tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D17313