Fixed Docker on Docker issue #15

Job	Run time
Build aarch64-unknown-linux-gnu	2m 46s
Build x86_64-apple-darwin	2m 27s
Build x86_64-unknown-linux-gnu	2m 58s
Build aarch64-apple-darwin	1m 38s
Build x86_64-pc-windows-msvc	3m 33s
Build thumbv7m-none-eabi	2m 12s
Build armv7-linux-androideabi	4m 38s
Build armv7-unknown-linux-gnueabihf	4m 18s
release	-1s
	24m 29s

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