Alpine images need nsswitch.conf for DNS

Description

As per sgerrand/alpine-pkg-glibc/issues/4, glibc dependent programs need an /etc/nsswitch.conf to determine their name resolution order.

Current gitlab-runner alpine images doesn't include such a file, meaning /etc/hosts is ignored by glibc programs (i.e. gitlab-runner). This means --add-host option is ignored by gitlab-runner alpine containers.

Proposal

Include an /etc/nsswitch.conf file in the alpine images for gitlab-runner which prioritizes files (/etc/hosts) over external DNS; i.e.:

RUN echo 'hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4' > /etc/nsswitch.conf