Change image to use github.url to allow testing of social media images

[mgriffin-scottlogic]

Fixes #285

No post associated with change, though I did make a test one to prove that a new image will now appear in the LinkedIn Post Inspector.

Have you (please tick each box to show completion):

• Added your blog post to a single category?
• Added a brief summary for your post? Summaries should be roughly two sentences in length and give potential readers a good idea of the contents of your post.
• Checked that the build passes?
• Checked your spelling (you can use npm install followed by npx mdspell "**/{FILE_NAME}.md" --en-gb -a -n -x -t if that's your thing)
• Ensured that your author profile contains a profile image, and a brief description of yourself? (make it more interesting than just your job title!)
• Optimised any images in your post? They should be less than 100KBytes as a general guide.

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