Dockerize the build workflow on Travis CI for Linux

[okapies]

Run building Menoh on Linux using Docker container. Detailed design is written in .travis/README.md.

Note: Currently linux-x86_64 platform uses GCC 6.3.1 on CentOS 6. I'd like to use more older GCC for improving binary compatibility but I'm facing some problems. I plan to solve them later.

[okapies]

@msakai Could you please review it? Don't hesitate to reach me at online/offline if you have any questions. Thanks.

[msakai]

Sure!

[msakai]

Is it intended that ldd run outside docker?

[okapies]

Yes, but, as you pointed out, it is not useful and confusing. I'll fix it.

[msakai]

I see.

[msakai]

It failed with the following error:

Usage
  cmake [options] <path-to-source>
  cmake [options] <path-to-existing-build>
Specify a source directory to (re-)generate a build system for it in the
current working directory.  Specify an existing build directory to
re-generate its build system.
Run 'cmake --help' for more information.
bash: line 76: -DMKLDNN_INCLUDE_DIR=/home/travis/mkl-dnn-0.16/include: No such file or directory

This is because the build_command_prepend value is interpreted as "cov-configure --compiler /usr/bin/g++-7 --comptype g++ -- -march=native -fPIC -std=gnu++14 &&\n cmake\n -DMKLDNN_INCLUDE_DIR=\"$HOME/mkl-dnn-${MKLDNN_VERSION}/include\"\n -DMKLDNN_LIBRARY=\"$HOME/mkl-dnn-${MKLDNN_VERSION}/lib/libmkldnn.so\" .".

It seems that we need to write as follows (same indentation across lines):

                  build_command_prepend: >-
                      cov-configure --compiler /usr/bin/g++-7 --comptype g++ -- -march=native -fPIC -std=gnu++14 &&
                      cmake
                      -DMKLDNN_INCLUDE_DIR="$HOME/mkl-dnn-${MKLDNN_VERSION}/include"
                      -DMKLDNN_LIBRARY="$HOME/mkl-dnn-${MKLDNN_VERSION}/lib/libmkldnn.so" .

[okapies]

Good catch. Fixed.

[msakai]

If this build matrix entry is just for running Coverity Scan, you might want to skip script section.
How do you think?

[okapies]

I fixed to skip the build step and avoid running this job in branches other than coverity_scan. It seems to work as expected in the target branch.

[msakai]

Thanks! It looks 👌.

[msakai]

LGTM!

[okapies]

Thanks so much!