docs: add devcontainer db sharing guide

README.md

@@ -35,15 +35,32 @@ Achievements is a Visual Studio Code extension that allows you to track your pro
 - A lock prevents multiple VS Code instances from writing the database at the same time.
   - If another instance is using the DB, the extension runs in **read-only** mode: listeners are disabled and a status bar item indicates the state.
 
+### Sharing progress across devcontainers
+
+When VS Code connects to a devcontainer, `achievements.sqlite` is stored inside the container's filesystem, at a path determined by the extension ID: `~/.vscode-server/data/User/globalStorage/boxboxjason.achievements/achievements.sqlite`
+
+That path is deterministic across every devcontainer you connect to, so you can persist and share progress between containers (and across rebuilds) by mounting a host directory or named Docker volume at that exact location in `.devcontainer/devcontainer.json`:
+
+```jsonc
+{
+  "mounts": [
+    "source=${localEnv:HOME}/.achievements-data,target=/home/vscode/.vscode-server/data/User/globalStorage/boxboxjason.achievements,type=bind"
+  ]
+}
+```
+
+Adjust the remote user's home directory in `target` to match your container image (e.g. `/root` instead of `/home/vscode`), and use the same `source` across devcontainer configs to share one database between all of them.
+
+> **Note:** only open one devcontainer at a time against a shared database. The write lock file
+> lives under the container's own temp directory rather than next to the database, so two
+> containers open simultaneously against the same mounted database won't see each other's lock
+> and could both attempt to write at once.
+
 ### Network access
 
 By default, this extension makes **no network requests** and works fully offline.
 
-The only exception is the optional `achievements.checkOutdatedExtensions` setting (disabled
-by default). When enabled, it periodically sends the IDs of your installed (non-builtin)
-extensions to the VS Marketplace (`marketplace.visualstudio.com`) to check for outdated
-versions and award related achievements. Enable it only if you're comfortable sharing your
-installed-extension list with the Marketplace.
+The only exception is the optional `achievements.checkOutdatedExtensions` setting (disabled by default). When enabled, it periodically sends the IDs of your installed (non-builtin) extensions to the VS Marketplace (`marketplace.visualstudio.com`) to check for outdated versions and award related achievements. Enable it only if you're comfortable sharing your installed-extension list with the Marketplace.
 
 ## Extension Commands