[build]: change user name to lower case when used in sonic-slave tag (#6319)

sonic-slave tag only allows all lower case. In case the user
name is mixed case, we need to change user name to all lower case.

Signed-off-by: Guohan Lu <lguohan@gmail.com>

Makefile.work

@@ -45,6 +45,7 @@ SHELL = /bin/bash
 
 USER := $(shell id -un)
 PWD := $(shell pwd)
+USER_LC := $(shell echo $(USER) | tr A-Z a-z)
 
 comma := ,
 
@@ -100,7 +101,7 @@ SLAVE_BASE_IMAGE = $(SLAVE_DIR)
 else
 SLAVE_BASE_IMAGE = $(SLAVE_DIR)-$(CONFIGURED_ARCH)
 endif
-SLAVE_IMAGE = $(SLAVE_BASE_IMAGE)-$(USER)
+SLAVE_IMAGE = $(SLAVE_BASE_IMAGE)-$(USER_LC)
 
 # Generate the version control build info
 $(shell SONIC_VERSION_CONTROL_COMPONENTS=$(SONIC_VERSION_CONTROL_COMPONENTS) \