build: improve error messages for docker driver #2667

Job	Run time
buildkit-edge	2m 41s
prepare-binaries	6s
prepare-test	2m 53s
binaries (darwin/amd64)	2m 36s
binaries (darwin/arm64)	2m 10s
binaries (linux/amd64)	2m 40s
binaries (linux/arm/v6)	2m 16s
binaries (linux/arm/v7)	2m 25s
binaries (linux/arm64)	2m 52s
binaries (linux/ppc64le)	2m 10s
binaries (linux/riscv64)	2m 58s
binaries (linux/s390x)	2m 37s
binaries (windows/amd64)	2m 37s
binaries (windows/arm64)	2m 3s
test (docker, ./tests)	3m 55s
test (docker\+containerd, ./tests)	4m 44s
test (docker-container, ./tests)	3m 42s
test (remote, ./tests)	4m 29s
test (./..., 1)	5m 2s
release	25s
bin-image	0s
	55m 21s

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