Default DATA_DIR in entrypoint to match VOLUME in the Dockerfile

Details are included in RELEASING.md for inclusion in the next set of
release notes.

RELEASING.md

@@ -19,3 +19,55 @@
 1. Commit that change with comment "Bump to -pre version".
 1. Run `git push upstream`
 1. Navigate to the tag in the GitHub releases UI and create a release with general comments about the changes in the release
+
+---
+
+For the next release, include the folowing in the release notes:
+
+Running the Docker image for this server without specifying DATA_DIR
+defaulted to storing the server data in
+`/var/lib/taskchampion-sync-server`. However, the Dockerfile only
+specifies that the subdirectory `/var/lib/taskchampion-sync-server/data`
+is a VOLUME. This change fixes the default to match the VOLUME, putting
+the server data on an ephemeral volume or, if a `--volume
+$NAME:/var/lib/taskchampion-sync-server/data` argument is provided to
+`docker run`, in a named volume.
+
+Before this commit, with default settings the server data is stored in
+the container's ephemeral writeable layer. When the container is killed,
+the data is lost. This issue does not affect deployments with `docker
+compose`, as the compose configuration specifies a correct `DATA_DIR`.
+
+You can determine if your deployment is affected as follows. First,
+determine the ID of the running server container, `$CONTAINER`. Examine
+the volumes for that container:
+
+```shell
+$ docker container inspect $CONTAINER | jq '.[0].Config.Volumes'
+{
+  "/var/lib/task-champion-sync-server/data": {}
+}
+```
+
+Next, find the server data, in a `.sqlite3` file:
+
+```shell
+$ docker exec $CONTAINER find /var/lib/taskchampion-sync-server
+/var/lib/taskchampion-sync-server
+/var/lib/taskchampion-sync-server/data
+/var/lib/taskchampion-sync-server/taskchampion-sync-server.sqlite3
+```
+
+If the data is not in a directory mounted as a volume, then it is
+ephemeral. To copy the data out of the container:
+
+```shell
+docker cp $CONTAINER:/var/lib/taskchampion-sync-server/taskchampion-sync-server.sqlite3 /tmp
+```
+
+You may then upgrade the image and use `docker cp` to copy the data back
+to the correct location, `/var/lib/taskchampion-sync-server/data`.
+
+Note that, as long as all replicas are fully synced, the TaskChampion
+sync protocol is resilient to loss of server data, so even if the server
+data has been lost, `task sync` may continue to work.

docker-entrypoint.sh

@@ -2,7 +2,7 @@
 set -e
 echo "starting entrypoint script..."
 if [ "$1" = "/bin/taskchampion-sync-server" ]; then
-    : ${DATA_DIR:=/var/lib/taskchampion-sync-server}
+    : ${DATA_DIR:=/var/lib/taskchampion-sync-server/data}
     export DATA_DIR
     echo "setting up data directory ${DATA_DIR}"
     mkdir -p "${DATA_DIR}"