Add centos7 binaries

[ben-childs-docusign]

Address issue #134

Adds docker image and build script for the centos7 binaries.

Adds the centos7 binary to the nuget package for .net.

Tested consuming the package in a .net project
ran dotnet publish -r centos.7-x64 and verified that the new binary was copied.
ran dotnet publish -r linux-x64 and verified that the old binary was copied.

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[ben-childs-docusign]

This was a way to build easily locally on any machine w/ docker installed.
This should also work on azure-pipelines build agents but I did not hook that up here.

[SergeiAlonichau]

Hi Ben,

Thank you for the PR. Did you test how will consumer of the package in .net know which DLL to load or where to load it from?

Thanks!
-Sergei

[SergeiAlonichau]

Also could you please sign the required CLA above?

[ben-childs-docusign]

Hi Sergei
I'm waiting to receive approval to sign the CLA hopefully I can to that soon.
The dll from the centos.7 folder gets published when this command is run:
dotnet publish -r centos.7-x64
If you publish for linux-x64 you get the generic version.