Build the box environment in the core, and reach the proxy control channel by URL

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Part of #109 (step 2).

Changes

• The core builds the full environment of a box as a plain record, in boxEnv of the new orchestrator/src/boxenv.ts:
  the harness variables, the credential placeholders, the git identity, TERM, the proxy variables and the CA
  variables. Empty values are dropped there. Before, credentialEnv in docker.ts built the placeholders and the
  identity, and boxEnv in docker.ts added the rest and made the KEY=value list. createContainer now only converts
  the record to the list. The field caCertificate of CreateContainerSpec is gone; the CA is part of the record.
• The proxy URL comes from the runtime: proxyUrl(cfg) in docker.ts makes it from EGRESS_PROXY_ALIAS and
  EGRESS_PROXY_PORT. The core passes it into boxEnv.
• The orchestrator reaches the control channel of the proxy at the URL EGRESS_CONTROL_URL (default
  http://boxes-egress-proxy:3129, must be an http: URL). Before, it was built from EGRESS_PROXY_CONTAINER and
  EGRESS_CONTROL_PORT. EGRESS_CONTROL_PORT is gone. EGRESS_PROXY_CONTAINER stays for the attachment of the proxy
  to the box networks and for the image footer.
• doc/environment.md describes EGRESS_CONTROL_URL instead of EGRESS_CONTROL_PORT.

Breaking change: a deployment that sets EGRESS_CONTROL_PORT must set EGRESS_CONTROL_URL instead. The old
variable is ignored without an error.

Tests

• Orchestrator: tsc --noEmit is clean, 881 tests passed, 1 skipped (as before). The boxEnv tests moved from
  docker.test.ts to boxenv.test.ts with the same assertions. The egress tests set EGRESS_CONTROL_URL; before, they
  set EGRESS_PROXY_CONTAINER to 127.0.0.1 to reach a local test server. The config test checks the new default.
• The URL validation was checked by hand: https://x:1 and a non-URL are rejected, an empty value gives the default.

Not tested against a real Docker daemon or a real proxy.

Notes for the next steps

• Step 7, the Runtime interface. proxyUrl(cfg) in docker.ts becomes Runtime.proxyUrl(). EGRESS_PROXY_ALIAS
  and EGRESS_PROXY_PORT are then details of the Docker runtime. boxEnv in boxenv.ts is the record of section 7 of
  the concept; the Docker runtime converts it with the one line in createContainer. EGRESS_PROXY_CONTAINER is used
  by the proxy attachment in docker.ts and the image footer in images.ts; both move behind the interface.
• Step 9, Kubernetes. The record of boxEnv becomes the env list of the pod spec without a change. The URL
  EGRESS_CONTROL_URL then names the proxy Service, for example http://boxes-egress-proxy:3129. The orchestrator joins
  the path with new URL(path, EGRESS_CONTROL_URL), so the URL must have no path of its own.