Updated boost url for base docker container

[Navya-Verma11]

Pull Request Template

Description

Current Docker image was using Bintray for downloading boost files which has been already retired. I have changed it to Jfrog that is currently being used by boost. It was resulting in broken builds due to 404.

Fixes # (issue)
#112

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

I have used the updated Dockerfile to successfully build the base image.
I used
docker build -f Dockerfile.base.ubuntu.18.04 --build-arg NUM_CPU=4 -t moja/baseimage:ubuntu-18.04 .

Additional Context (Please include any Screenshots/gifs if relevant)

...

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings
• I have tagged the reviewers in a comment below incase my pull request is ready for a review
• I have signed the commit message to agree to Developer Certificate of Origin (DCO) (to certify that you wrote or otherwise have the right to submit your contribution to the project.) by adding "--signoff" to my git commit command.

[Navya-Verma11]

@mfellows, @malfrancis, could you please review the PR.