[explorer] Use the whole tree as context, use locked versions

Summary: Now that the chronik client is in the tree we will have to enlarge the docker context. We can then bring back the dependency locking.

Test Plan:

@bot build-explorer preview-explorer

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17474