feat: use buildx for multi-arch docker image #42

Job	Run time
build (1.21.x, GOARCH=amd64 GOOS=linux, gno)	10s
build (1.21.x, GOARCH=wasm GOOS=js, gno)	26s
build (1.22.x, GOARCH=amd64 GOOS=linux, gno)	12s
build (1.22.x, GOARCH=wasm GOOS=js, gno)	30s
test (1.21.x, _test.cmd)	1m 16s
test (1.21.x, _test.pkg)	44s
test (1.21.x, _test.gnolang.native)	26s
test (1.21.x, _test.gnolang.stdlibs)	47s
test (1.21.x, _test.gnolang.realm)	36s
test (1.21.x, _test.gnolang.pkg0)	1m 39s
test (1.21.x, _test.gnolang.pkg1)	9m 37s
test (1.21.x, _test.gnolang.pkg2)	3m 56s
test (1.21.x, _test.gnolang.other)	23s
test (1.22.x, _test.cmd)	1m 16s
test (1.22.x, _test.pkg)	39s
test (1.22.x, _test.gnolang.native)	21s
test (1.22.x, _test.gnolang.stdlibs)	47s
test (1.22.x, _test.gnolang.realm)	32s
test (1.22.x, _test.gnolang.pkg0)	1m 39s
test (1.22.x, _test.gnolang.pkg1)	9m 43s
test (1.22.x, _test.gnolang.pkg2)	3m 57s
test (1.22.x, _test.gnolang.other)	28s
upload-coverage	18s
	40m 22s

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