[Cashtab] Also copy explorer in Dockerfile

Summary:
CI builds are failing with an error suggesting that ./build-wasm cannot run without /web/explorer/ being in place. Looks like it could be related to D17292

Now that the explorer is in the workspace of the root Cargo.toml, we need it available for ./build-wasm

Test Plan: docker build -f cashtab.Dockerfile -t cashtab_local .

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17315