dockerfile: use glibc to build riscv64 #1606

Job	Run time
prepare	1s
test / prepare	9m 27s
test / run (containerd, ./frontend/dockerfile, integration)	7m 48s
test / run (containerd, ./frontend/dockerfile, dockerfile)	9m 4s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	9m 39s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	11m 56s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	8m 49s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	9m 3s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	7m 16s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	16m 4s
test / run (oci, ./frontend/dockerfile, integration)	8m 15s
test / run (oci, ./frontend/dockerfile, dockerfile)	10m 48s
test / run (oci-rootless, ./frontend/dockerfile, integration)	8m 14s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	7m 23s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	7m 3s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	8m 19s
image (mainline)	7m 41s
image (labs)	8m 23s
release	0s
	2h 35m 13s

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