Staging environments, from CLI to proxy host

Added

• Staging deployments. A new service brings an application up in containers on demand, creating the networks and named volumes it needs rather than expecting them to exist.
• Every deployment gets a proxy host. Proxy hosts are created and removed as deployments come and go, so a staging environment is reachable without anyone editing a proxy by hand.
• A command line tool for staging. A first set of commands and configuration files makes it possible to drive staging deployments from a terminal.
• Environment variables come from CI. A deployment fetches its environment variables from CI and hands them to the containers it starts, instead of carrying its own copy.
• Pick the runner tag and environment when triggering a build. Triggering a CI build now accepts a runner tag and an environment, both from the command line and from a staging deployment.

Improved

• Containers start even when a port is taken. Port allocation handles the case where another container already holds the port it wanted, so a deployment comes up instead of failing at startup.
• Leftover containers get cleaned up. Containers are labelled when they are created, which lets orphaned ones be found by label and removed.