Uses official shellcheck Docker image for linting
Uses official shellcheck Docker image for linting

The Dockerfile is built from "scratch" and contains only the shellcheck
binary. Therefore we still have to run `find` outside the container and
pass in the filepaths relative to the `/mnt` directory inside the
container, which is also set as the working directory.

Also excluding SC1091 since the container won't have any of the included
files. Better for cross-platform testing anyway, since those checks
would have passed on Linux developer workstations, but failed on macOS
anyway.

Relevant resources:

  * https://hub.docker.com/r/koalaman/shellcheck/
  * https://github.com/koalaman/shellcheck

heartsucker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,garrettr/securedrop,conorsch/securedrop,micahflee/securedrop,heartsucker/securedrop,micahflee/securedrop,conorsch/securedrop,conorsch/securedrop,garrettr/securedrop,ehartsuyker/securedrop,garrettr/securedrop,heartsucker/securedrop,micahflee/securedrop,garrettr/securedrop,ehartsuyker/securedrop,micahflee/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop