fix: surface docker's error when it rejects a .deva mount

A .deva VOLUME docker refuses made deva.sh die silently -- "Creating
persistent container: <name>" and nothing else. Under set -euo pipefail
a failing command-substitution assignment aborts AT the assignment, so
docker_exit=$? and the entire error block below it were dead code on
the one path they exist for.

Keep the assignment left of `||` to suspend set -e and capture the real
exit; the existing handler then prints docker's own stderr.

Repro: .deva with VOLUME=/tmp:/mnt/bad:rx
  before: exit 125, no message
  after:  exit 1, "docker: Error response from daemon: invalid mode: rx"

The ephemeral path was already safe (if ! docker ...).

Closes #484

Author: lroolle

deva.sh

@@ -3950,8 +3950,12 @@ if [ "$EPHEMERAL_MODE" = false ]; then
     else
         # Container doesn't exist - try to create it
         echo "Creating persistent container: $CONTAINER_NAME"
-        error_output=$(docker "${DOCKER_ARGS[@]}" tail -f /dev/null 2>&1)
-        docker_exit=$?
+        # `|| docker_exit=$?` is load-bearing: under `set -e` a failing
+        # command-substitution assignment aborts the script AT the assignment,
+        # so the error handling below would be dead code on the exact path it
+        # exists for. Keep the assignment left of `||` to suspend `set -e`.
+        docker_exit=0
+        error_output=$(docker "${DOCKER_ARGS[@]}" tail -f /dev/null 2>&1) || docker_exit=$?
         if [ $docker_exit -ne 0 ]; then
             # Check if specifically a name collision (concurrent run)
             if echo "$error_output" | grep -qE 'already in use|Conflict'; then