Fix missing shebang in failover shell scripts

Add shebang line to failover scripts to fix RPM build warnings.

During RPM package build, rpmbuild reports warnings about executable
files without shebang lines. This causes the build process to remove
the executable bit from these scripts.

The following scripts are fixed:
- master_check_back.sh
- standby_check_back.sh
- segment_check_back.sh
- segment_all_down.sh
- docker_master_check_back.sh
- docker_standby_check_back.sh
- docker_segment_check_back.sh

Use "#!/usr/bin/env bash" for better cross-platform compatibility.

See: #1445

Author: tuhaihe

src/bin/gpfts/failover/docker_master_check_back.sh

@@ -1,3 +1,4 @@
+#!/usr/bin/env bash
 
 if !type gpssh >/dev/null 2>&1; then 
     echo "[FATAL]:- gpssh not exist, script Exits!"

src/bin/gpfts/failover/docker_segment_check_back.sh

@@ -1,4 +1,4 @@
-
+#!/usr/bin/env bash
 
 if !type gpssh >/dev/null 2>&1; then 
     echo "[FATAL]:- gpssh not exist, script Exits!"

src/bin/gpfts/failover/docker_standby_check_back.sh

@@ -1,4 +1,4 @@
-
+#!/usr/bin/env bash
 
 if !type gpssh >/dev/null 2>&1; then 
     echo "[FATAL]:- gpssh not exist, script Exits!"

src/bin/gpfts/failover/master_check_back.sh

@@ -1,3 +1,4 @@
+#!/usr/bin/env bash
 
 if !type gpssh >/dev/null 2>&1; then 
     echo "[FATAL]:- gpssh not exist, script Exits!"

src/bin/gpfts/failover/segment_all_down.sh

@@ -1,3 +1,4 @@
+#!/usr/bin/env bash
 
 if !type pg_ctl >/dev/null 2>&1; then 
     echo "[FATAL]:- pg_ctl not exist, script Exits!"

src/bin/gpfts/failover/segment_check_back.sh

@@ -1,3 +1,4 @@
+#!/usr/bin/env bash
 
 if !type gprecoverseg >/dev/null 2>&1; then 
     echo "[FATAL]:- gprecoverseg not exist, script Exits!"

src/bin/gpfts/failover/standby_check_back.sh

@@ -1,4 +1,4 @@
-
+#!/usr/bin/env bash
 
 if !type gpssh >/dev/null 2>&1; then 
     echo "[FATAL]:- gpssh not exist, script Exits!"