Update alpine Docker tag to v3.21.2 #26

Job	Run time
Supported processor platforms	11s
Lint Dockerfile	11s
Build linux/amd64 image	35s
Build linux/arm image	1m 1s
Build linux/arm64 image	1m 2s
Build linux/386 image	42s
Build linux/ppc64le image	1m 7s
Build linux/riscv64 image	1m 0s
Build linux/s390x image	1m 25s
Scan for vulnerabilities (linux/amd64)	26s
Scan for vulnerabilities (linux/arm)	17s
Scan for vulnerabilities (linux/arm64)	25s
Scan for vulnerabilities (linux/386)	25s
Scan for vulnerabilities (linux/ppc64le)	20s
Scan for vulnerabilities (linux/riscv64)	18s
Scan for vulnerabilities (linux/s390x)	18s
Test linux/amd64	15s
Test linux/arm	17s
Test linux/arm64	30s
Test linux/386	23s
Test linux/ppc64le	22s
Test linux/riscv64	23s
Test linux/s390x	20s
Push	0s
	12m 13s

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