[sdk] Tweak SDK Docker image.

When mounting `/home`, gosu may inadvertantly write to the host's
because of `useradd`.

coq-jslib/build/sdk/toolkit.js

@@ -69,12 +69,14 @@ class DockerTool extends Phase {
 
     mounts(cfg) {
         var cwd = process.cwd(),
-            mnt = cwd.match(/[/][^/]+/)[0],
+            mnt = cwd.match(/^[/][^/]+/)?.[0],
             coqlib = cfg.coqlib;
     
         if (!mnt)
             console.warn(`[${ME}-sdk] cannot detect working directory root for '${cwd}';\n` +
                          `            this will probably not work.`)
+        if (mnt.match(/^[/](Users|home)$/))
+            mnt = cwd.match(/^[/][^/]+[/][^/]+/)?.[0] ?? mnt; /* this is highly heuristic */
     
         return [mnt, coqlib].filter(x => x).map(d => `-v${d}:${d}`);
     }

etc/docker/gosu-entrypoint.sh

@@ -18,11 +18,11 @@
 USER_ID=${LOCAL_USER_ID:-9001}
 GROUP_ID=${LOCAL_GROUP_ID:-$USER_ID}
 
-if [ x"$LOCAL_USER_ID" != x ] ; then
-  echo "Starting with UID : $USER_ID, GID: $GROUP_ID"
+if [ -t 2 ] && [ x"$LOCAL_USER_ID" != x ] ; then
+  echo "# Starting with UID : $USER_ID, GID: $GROUP_ID" >&2
 fi
-groupadd -g $GROUP_ID thegroup
-useradd --shell /bin/bash -u $USER_ID -g thegroup -o -c "" -m user
-export HOME=/home/user
+groupadd -g $GROUP_ID ateam
+useradd --shell /bin/bash -u $USER_ID -g ateam -o -c "" -m jscoq
+export HOME=/home/jscoq
 
-exec /usr/sbin/gosu user:thegroup "${@:-bash}"
\ No newline at end of file
+exec /usr/sbin/gosu jscoq:ateam "${@:-bash}"