[Cashtab] Dockerfile lint

Summary:
Always get a lint error when i test this, apparently should not capitalize this

Depends on D17315

Test Plan: docker build -f cashtab.Dockerfile -t cashtab_local . and no lint error

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17316