CI: Consolidate Dockerfiles

[leezu]

Description

This PR shows how to get rid of duplicate Dockerfiles and the install/X.sh code deduplication strategy on the example of CentOS7 Dockerfiles. Instead, we use a single templated Dockerfile with a multiple targets for different scenarios. The template arguments and respective targets are declared in docker-compose.yml.

This has multiple benefits:

1. Having a single Dockerfile instead of a number of scripts is easier to understand and maintain
2. Copying scripts into the Dockerfile as done in the previous approach prevents usage of the https://hub.docker.com/u/mxnetci/ cache on developer machines, as the COPY instruction invalidates the cache if the local repository does not match the repository used on CI (eg. development branch instead of master branch). If the cache is invalidated, developers must wait for long docker build time when attempting to reproduce certain CI builds (such as website build, cc @aaronmarkham) or using the docker image to perform the staticbuild.
3. Having fewer steps in the Dockerfile improves the build speed.

[mxnet-bot]

Hey @leezu , Thanks for submitting the PR
All tests are already queued to run once. If tests fail, you can trigger one or more tests again with the following commands:

• To trigger all jobs: @mxnet-bot run ci [all]
• To trigger specific jobs: @mxnet-bot run ci [job1, job2]

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CI supported jobs: [windows-gpu, windows-cpu, miscellaneous, website, centos-gpu, unix-gpu, edge, sanity, centos-cpu, clang, unix-cpu]

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Note:
Only following 3 categories can trigger CI :PR Author, MXNet Committer, Jenkins Admin.
All CI tests must pass before the PR can be merged.

[marcoabreu]

That looks really neat!