docs: compose environment and built-from-source services (#7)

README.md

@@ -20,7 +20,8 @@ runs is exactly what was pulled. The log streams to the browser while it runs; j
 history is kept in Postgres. A service without a tag (`repo@digest`), a plain container,
 and the updater's own services cannot be updated, and the UI says why. A bare image
 (`image: postgres`, no tag and no digest) counts as `postgres:latest`; only a reference
-with a digest and no tag (`repo@sha256:...`) "follows no tag". One update at a
+with a digest and no tag (`repo@sha256:...`) "follows no tag". Compose services with a `build:` section are shown but never updated
+("built from source (build:)"). One update at a
 time per service, and one job at a time per agent.
 
 ## Deploying the master
@@ -46,6 +47,11 @@ the agent's token (shown once) and the master's key fingerprint; the same layout
 | `DOCKER_CONFIG` | no | — | Directory with a `config.json` holding registry credentials. Only inline `auths` entries work; `credsStore`/`credHelpers` are not available in the container. |
 | `LOG_LEVEL` | no | `info` | `debug` / `info` / `warn` / `error`. |
 
+`docker compose` runs inside the agent with only `PATH`, `HOME` and the `DOCKER_*`
+variables. Variables a stack uses for interpolation must come from the stack's own `.env`
+file or an `--env-file` (which the agent honours), never from the agent container's
+environment.
+
 ### Registry credentials
 
 With `DOCKER_CONFIG` set, Compose jobs pull with the same credentials as `docker compose`.