Handle missing gitaly/workhorse in 'gdk update'

Closes #181

See merge request !238

Note npm requirement for Debian

See merge request !236

Refer to gitlab development guidelines in docs

See merge request !233

Fix typo in socket name

See merge request !234

Configure gitaly in Procfile.example

See merge request !232

Build gitaly the same way as gitlab-workhorse

See merge request !231

Setup git-access-daemon within the GDK

See merge request !228

Install nodejs-legacy and npm on Ubuntu

Closes #174

See merge request !230

Update the NGINX configuration to support websockets

https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/83 added websocket support to workhorse. The bundled nginx configuration needs updating to support this.

https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1146 adds support to omnibus-gitlab in a similar way.

See merge request !226

prune unneeded packages on update

See merge request !229

Bump Ruby version in Dockerfile for GDK test image

Needed after https://gitlab.com/gitlab-org/gitlab-development-kit/merge_requests/217, because Ruby version in GitLab changed recently.

See merge request !225

Automated GDK tests

Closes #167

See merge request !217

Fix openldap download URL and add SHA256 check

The SHA256 check will catch corrupt/incomplete downloads.

Fixes https://gitlab.com/gitlab-org/gitlab-development-kit/issues/177

See merge request !224

The SHA256 check will catch corrupt/incomplete downloads.

Document foreman thor reserved word issue

Fixes https://gitlab.com/gitlab-org/gitlab-development-kit/issues/175

See merge request !223