[Docker] Updated bash.sh for MacOS compatibility

MacOS has an older version of bash that handles arrays slightly
differently.  All instances of array expansion `"${ARRAY[@]}"` should
instead be written as `${ARRAY[@]+"${ARRAY[@]}"}`.  Otherwise, `set -u`
will erroneously complain about an undefined variable. See
https://stackoverflow.com/a/61551944 for details.

Even though this is an older version of bash (observed in version
3.2.57), this is the last major version available under GPLv2 and is
therefore the default version on MacOSX.  At some point, the
`docker/bash.sh` could be migrated to python for ease of
maintenance/testing.

docker/bash.sh

@@ -222,7 +222,7 @@ if [[ -z "${DOCKER_IMAGE_NAME}" ]]; then
     show_usage >&2
 fi
 
-if [[ "${COMMAND[@]}" = bash ]]; then
+if [[ ${COMMAND[@]+"${COMMAND[@]}"} = bash ]]; then
     INTERACTIVE=true
     USE_NET_HOST=true
 fi
@@ -297,7 +297,7 @@ if ${INTERACTIVE}; then
 fi
 
 # Expose external directories to the docker container
-for MOUNT_DIR in "${MOUNT_DIRS[@]}"; do
+for MOUNT_DIR in ${MOUNT_DIRS[@]+"${MOUNT_DIRS[@]}"}; do
     DOCKER_MOUNT+=( --volume "${MOUNT_DIR}:${MOUNT_DIR}" )
 done
 
@@ -369,20 +369,20 @@ echo "REPO_DIR: ${REPO_DIR}"
 echo "DOCKER CONTAINER NAME: ${DOCKER_IMAGE_NAME}"
 echo ""
 
-echo "Running '${COMMAND[@]}' inside ${DOCKER_IMAGE_NAME}..."
+echo Running \'${COMMAND[@]+"${COMMAND[@]}"}\' inside ${DOCKER_IMAGE_NAME}...
 
 DOCKER_CMD=(${DOCKER_BINARY} run
-            "${DOCKER_FLAGS[@]}"
-            "${DOCKER_ENV[@]}"
-            "${DOCKER_MOUNT[@]}"
-            "${DOCKER_DEVICES[@]}"
+            ${DOCKER_FLAGS[@]+"${DOCKER_FLAGS[@]}"}
+            ${DOCKER_ENV[@]+"${DOCKER_ENV[@]}"}
+            ${DOCKER_MOUNT[@]+"${DOCKER_MOUNT[@]}"}
+            ${DOCKER_DEVICES[@]+"${DOCKER_DEVICES[@]}"}
             "${DOCKER_IMAGE_NAME}"
             bash --login /docker/with_the_same_user
-            "${COMMAND[@]}"
+            ${COMMAND[@]+"${COMMAND[@]}"}
            )
 
 if ${DRY_RUN}; then
-    echo "${DOCKER_CMD[@]}"
+    echo ${DOCKER_CMD[@]+"${DOCKER_CMD[@]}"}
 else
-    "${DOCKER_CMD[@]}"
+    ${DOCKER_CMD[@]+"${DOCKER_CMD[@]}"}
 fi