Permit '=' separator and '[ipv6]' in --add-host #2979

Job	Run time
prepare-binaries	5s
buildkit-edge	1m 17s
prepare-test	1m 23s
binaries (darwin/amd64)	1m 25s
binaries (darwin/arm64)	1m 34s
binaries (linux/amd64)	1m 33s
binaries (linux/arm/v6)	1m 20s
binaries (linux/arm/v7)	1m 26s
binaries (linux/arm64)	1m 22s
binaries (linux/ppc64le)	1m 20s
binaries (linux/riscv64)	1m 25s
binaries (linux/s390x)	1m 31s
binaries (windows/amd64)	1m 27s
binaries (windows/arm64)	1m 28s
test (docker, ./tests)	3m 28s
test (docker\+containerd, ./tests)	3m 8s
test (docker-container, ./tests)	2m 38s
test (remote, ./tests)	2m 35s
test (./..., 1)	2m 52s
bin-image	0s
release	15s
	33m 32s

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