fix docker image user agent

[chris48s]

Closes #7573

${GITHUB_SHA::7} is a bash subscript syntax, so we need to run it with bash and write it to a var

[shields-ci]

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[calebcartwright]

Can't think of any easy way to readily test this, but it looks like the right change to me anyway

[chris48s]

Can't think of any easy way to readily test this

Yeah this was a bit of a PITA to figure out.
If you're interested, the commits/builds I used to work this out were https://github.com/badges/shields/compare/ua-debug 😆