Update alpine Docker tag to v3.21.2 #12

Job	Run time
Supported processor platforms	20s
Lint Dockerfile	5s
Push	0s
Build linux/amd64 image	35s
Build linux/arm image	1m 2s
Build linux/arm64 image	52s
Build linux/386 image	39s
Build linux/ppc64le image	58s
Build linux/riscv64 image	51s
Build linux/s390x image	1m 19s
Scan for vulnerabilities (linux/amd64)	25s
Scan for vulnerabilities (linux/arm)	16s
Scan for vulnerabilities (linux/arm64)	18s
Scan for vulnerabilities (linux/386)	18s
Scan for vulnerabilities (linux/ppc64le)	23s
Scan for vulnerabilities (linux/riscv64)	17s
Scan for vulnerabilities (linux/s390x)	23s
	9m 1s

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