Improve Docker build speed in CI

[wangkuiyi]

I see that in the .travis.yaml file of SQLFlow, there is a line

https://github.com/sql-machine-learning/sqlflow/blob/a033b42926e4d0ccf63c044e414aa99eb9dfcfaf/.travis.yml#L38

It seems that our .travis.yaml doesn't have such a line

elasticdl/.travis.yml

Lines 25 to 31 in 3a157c1

	jobs:
	include:
	- stage: precommit
	name: "Pre-commit Check"
	script:
	- docker build -t elasticdl:dev -f elasticdl/docker/Dockerfile.dev .
	- docker run --rm -it -v $PWD:/work -w /work elasticdl:dev bash -c "pre-commit run --files $(find elasticdl/python model_zoo setup.py scripts/ -name '*.py' -print0 | tr '\0' ' ') $(find elasticdl/pkg -name '*.go' -print0 | tr '\0' ' ')"

If we add such a trick, would it accelerate the CI?

[typhoonzero]

If the current Dockerfile got many steps that will perform exactly the same on every build, caching will save a lot of time, yet adding --cache-from will need to pull the image first which may be slow.