backend/docker: add ca-certificates to container
backend/docker: add ca-certificates to container

This is needed so the server can verify Stripe's TLS certificate when it makes a
POST request to its HTTPS API.

Signed-off-by: Austin Seipp <48c905ac3851f382b914247d97dd93273d5ff555@pobox.com>

thoughtpolice/haskell-donate,thoughtpolice/haskell-donate