Update alpine Docker tag to v3.21.3 #27

Job	Run time
Lint Dockerfile	5s
Supported processor platforms	20s
Build linux/amd64 image	39s
Build linux/arm image	1m 6s
Build linux/arm64 image	1m 16s
Build linux/386 image	34s
Build linux/ppc64le image	1m 13s
Build linux/riscv64 image	1m 10s
Build linux/s390x image	1m 38s
Scan for vulnerabilities (linux/amd64)	20s
Scan for vulnerabilities (linux/arm)	18s
Scan for vulnerabilities (linux/arm64)	18s
Scan for vulnerabilities (linux/386)	19s
Scan for vulnerabilities (linux/ppc64le)	25s
Scan for vulnerabilities (linux/riscv64)	25s
Scan for vulnerabilities (linux/s390x)	19s
Test linux/amd64	14s
Test linux/arm	24s
Test linux/arm64	18s
Test linux/386	14s
Test linux/ppc64le	24s
Test linux/riscv64	16s
Test linux/s390x	17s
Push	0s
	12m 32s

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