src-expose - support sharing read-only repositories

Created by: abitrolly

Looks like src-expose is unable to share repo that it can not write to.

Steps to reproduce:

1. podman run -v "${PWD}:/var/opt/localrepo":Z,ro sourcegraph/src-expose:latest serve /var/opt/localrepo

Expected behavior:

See repo served with some defaults.

Actual behavior:

serve: 2020/06/25 07:50:42 listening on http://[::]:3434
serve: 2020/06/25 07:50:42 serving git repositories from /var/opt/localrepo
serve: 2020/06/25 07:50:42 configuring repo at /var/opt/localrepo/.git: updating server info: error: unable to update info/refs: Read-only file system
error: unable to update objects/info/packs: Read-only file system
warning: unable to unlink 'info/rev-cache': Read-only file system
: exit status 1
serve: 2020/06/25 07:50:42 error encountered on /var/opt/localrepo/.sourcegraph/data/cache: open /var/opt/localrepo/.sourcegraph/data/cache: permission denied
serve: 2020/06/25 07:50:42 error encountered on /var/opt/localrepo/.sourcegraph/data/grafana/png: open /var/opt/localrepo/.sourcegraph/data/grafana/png: permission denied
serve: 2020/06/25 07:50:42 error encountered on /var/opt/localrepo/.sourcegraph/data/postgresql: open /var/opt/localrepo/.sourcegraph/data/postgresql: permission denied
serve: 2020/06/25 07:50:42 error encountered on /var/opt/localrepo/.sourcegraph/data/zoekt/index/.indexserver.tmp/tmpfriend-30-968347374: open /var/opt/localrepo/.sourcegraph/data/zoekt/index/.indexserver.tmp/tmpfriend-30-968347374: permission denied