fix: [nomad-prep] P10 — audit lib/ + compose for docker-backend-isms (#797)

Sites touched:
- lib/generators.sh: WOODPECKER_BACKEND_DOCKER_NETWORK now reads from
  ${WOODPECKER_CI_NETWORK:-disinto_disinto-net} so nomad jobspecs can
  override the compose-generated network name.
- lib/forge-setup.sh: bare-mode _forgejo_exec() and setup_forge() use
  ${FORGEJO_CONTAINER_NAME:-disinto-forgejo} instead of hardcoding the
  container name. Compose mode is unaffected (uses service name).

Documented exceptions (container_name directives in generators.sh
compose template output): these define names inside docker-compose.yml,
which is compose-specific output. Under nomad the generator is not used.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/generators.sh

@@ -318,7 +318,7 @@ services:
       WOODPECKER_AGENT_SECRET: ${WOODPECKER_AGENT_SECRET:-}
       WOODPECKER_GRPC_SECURE: "false"
       WOODPECKER_HEALTHCHECK_ADDR: ":3333"
-      WOODPECKER_BACKEND_DOCKER_NETWORK: disinto_disinto-net
+      WOODPECKER_BACKEND_DOCKER_NETWORK: ${WOODPECKER_CI_NETWORK:-disinto_disinto-net}
       WOODPECKER_MAX_WORKFLOWS: 1
     healthcheck:
       test: ["CMD", "wget", "-q", "--spider", "http://localhost:3333/healthz"]