test: use [[ ]] for conditional tests in the mooncake image scripts

Switch reportSummary.sh's empty-response guard to [[ ]] as requested in
review.

custom-entrypoint.sh needs its shebang changed along with the brackets.
It ran under #!/bin/sh, where /bin/sh is dash on the Debian-based
python:3.12-slim image, and dash has no [[ builtin: it resolves [[ as a
command name, fails with "[[: not found" and evaluates the condition as
false. Both guards in that script -- the unsupported-action check and the
FLUID_RUNTIME_CONFIG_PATH check -- would silently stop rejecting anything
while still exiting 0. The image ships bash, so the script now declares
it.

Author: btxu-db

test/gha-e2e/mooncake/image/custom-entrypoint.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 # Component entrypoint invoked by Fluid's CacheRuntime.
 # Usage: /custom-entrypoint.sh <master|worker|client> start
 
@@ -7,7 +7,7 @@ set -e
 ROLE="$1"
 ACTION="$2"
 
-if [ "$ACTION" != "start" ]; then
+if [[ "$ACTION" != "start" ]]; then
   echo "Error: unsupported action '$ACTION'"
   exit 1
 fi
@@ -27,7 +27,7 @@ case "$ROLE" in
 
   worker)
     # Read the runtime config JSON that Fluid mounts into the component pod.
-    if [ -z "$FLUID_RUNTIME_CONFIG_PATH" ] || [ ! -f "$FLUID_RUNTIME_CONFIG_PATH" ]; then
+    if [[ -z "$FLUID_RUNTIME_CONFIG_PATH" ]] || [[ ! -f "$FLUID_RUNTIME_CONFIG_PATH" ]]; then
       echo "Error: FLUID_RUNTIME_CONFIG_PATH not set or file not found"
       exit 1
     fi

test/gha-e2e/mooncake/image/reportSummary.sh

@@ -6,7 +6,7 @@ set -euo pipefail
 
 RAW=$(curl -s http://localhost:9003/metrics/summary)
 
-if [ -z "$RAW" ]; then
+if [[ -z "$RAW" ]]; then
   echo "Error: empty response from metrics endpoint" >&2
   exit 1
 fi