docs: keep the master's registry credentials private when running as nobody (#25)

README.md

@@ -37,7 +37,7 @@ socket is root on the host anyway.
 | `UPDATER_API_KEY` | no | — | Bearer token for `POST /update`. Without it the webhook answers `503` and the master logs `webhook_disabled`. |
 | `UPDATE_TIMEOUT` | no | `5m` | How long one `POST /update` waits for its auto jobs (Go duration). |
 | `POLL_INTERVAL` | no | `6h` | Time between registry polling cycles (Go duration, must be positive). The first cycle runs one minute after start. |
-| `DOCKER_CONFIG` | no | `~/.docker` | Directory with a `config.json` holding registry credentials for polling. Only inline `auths` entries work; see [Registry polling](#registry-polling). |
+| `DOCKER_CONFIG` | no | `~/.docker` | Directory with a `config.json` holding registry credentials for polling, readable by the master's user. Only inline `auths` entries work; see [Registry polling](#registry-polling). |
 
 The master's other settings (`DATABASE_URL`, `PUBLIC_URL`, `AGENT_URL`, `OIDC_*`,
 `ALLOWED_GROUP`, `AGENT_IMAGE`, `PORT`, `AGENT_PORT`, `LOG_LEVEL`) are shown in
@@ -147,9 +147,10 @@ multi-arch image the digest is the index digest, the same one `docker inspect` s
   "poll failed" badge on the overview and in the "Polling" row of the service page; other
   tags continue. A later success clears it.
 - **Credentials.** The master reads `$DOCKER_CONFIG/config.json` (a file written by
-  `docker login`), mounted read-only into the container and readable by the master's user (`65534` in the
+  `docker login`), mounted read-only into the container; without it registries are accessed
-  example stack); without it registries are accessed
+  anonymously. The directory and file must be readable by the master's user (`65534` in the
-  anonymously. Only inline `auths` entries are supported. A `credsStore` or `credHelpers`
+  example stack) without being world-readable: `chown -R 65534:65534` the directory and keep
+  `0700`/`0600`. Only inline `auths` entries are supported. A `credsStore` or `credHelpers`
   entry makes every lookup fail with `error getting credentials ... executable file not found
   in $PATH`, because the container has no credential helper binaries. The master logs
   `registry_credentials` at startup with the directory, `config_json_found` and
@@ -203,7 +204,7 @@ the agent's token (shown once) and the master's key fingerprint; the same layout
 | `AGENT_TOKEN` | yes | — | Token shown once when the agent was added. |
 | `SELF_ID` | no | hostname | Container ID or name of the agent itself; used to read its mounts. |
 | `UPDATE_TIMEOUT` | no | `5m` | Per-job timeout (Go duration). |
-| `DOCKER_CONFIG` | no | — | Directory with a `config.json` holding registry credentials, readable by the master's user (see the `config_json_readable` and `registry_credentials_unreadable` log fields). Only inline `auths` entries work; `credsStore`/`credHelpers` are not available in the container. |
+| `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_*`

deploy/master.stack.yml

@@ -13,10 +13,11 @@
 #   5. Optional, for registry polling of private images: put a config.json written by
 #      `docker login` (inline "auths" only; no credsStore) in /path/to/docker-config and
 #      uncomment DOCKER_CONFIG and the volumes entry below. The master runs as 65534:65534, so
-#      config.json must be readable by that user: `chmod 0644` it, or set `user:` below to the
+#      the directory and config.json must be readable by that user while staying private:
-#      file owner's uid:gid. The directory must exist on every
+#      `chown -R 65534:65534 /path/to/docker-config` (keep 0700/0600), or set `user:` below
-#      manager node (the master is placed on a manager), or Swarm rejects the task. Without
+#      to the owner's uid:gid. Do not make config.json world-readable; it holds registry
-#      it registries are accessed anonymously.
+#      passwords. The directory must exist on every manager node (the master is placed on a
+#      manager), or Swarm rejects the task. Without it registries are accessed anonymously.
 services:
   master:
     image: gitea.shcizo.se/shcizo/package-updater:latest