[BITCOIN-ABC CI] Added cargo build --locked CI to web/explorer && explorer.Dockerfile

Summary: Every time there is a change to any file within web/explorer, run cargo build --locked && cargo test

Test Plan: @bot build-explorer python3 contrib/teamcity/build-configurations.py build-explorer @bot preview-explorer

Reviewers: #bitcoin_abc, tobias_ruck, O1 Bitcoin ABC, Fabien

Reviewed By: #bitcoin_abc, tobias_ruck, O1 Bitcoin ABC, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17295