securedrop: add dev-shell target
securedrop: add dev-shell target

this runs bash in a bare dev-oriented docker environment

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