(core) persist logins across docker restarts by default

Summary:
Unless redis is configured, Grist will store information in a
grist-sessions.db file. For grist-core run using docker, this
file is not by default in the directory the documentation suggests
the user persist, and so is lost when Grist is updated. This
is an unexpected annoyance for users, and the configuration fix
is awkward to explain, so this diff changes the default.

Test Plan:
rebuilt image manually and verified it operated as
expected

Reviewers: georgegevoian

Differential Revision: https://phab.getgrist.com/D3217

Dockerfile

@@ -82,6 +82,7 @@ ENV GRIST_HOST=0.0.0.0
 ENV GRIST_SINGLE_PORT=true
 ENV GRIST_SERVE_SAME_ORIGIN=true
 ENV GRIST_DATA_DIR=/persist/docs
+ENV GRIST_INST_DIR=/persist
 ENV GRIST_SESSION_COOKIE=grist_core
 ENV TYPEORM_DATABASE=/persist/home.sqlite3
 EXPOSE 8484