Only use 2 path components for compose file

[zrhoffman]

This PR fixes some issues with paths in optional/docker-compose.traffic-portal-v2-test.yml that arose when it was moved to the optional directory. Apparently docker-compose always uses the directory that the first compose file specified with -f is in for the base of all relative paths.

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Which Traffic Control components are affected by this PR?

• CDN in a Box

What is the best way to verify this PR?

cd infrastructure/cdn-in-a-box
docker-compose -f docker-compose.yml -f optional/docker-compose.traffic-portal-v2.yml -f optional/docker-compose.traffic-portal-v2-test.yml build tpv2-e2e-test
docker-compose -f docker-compose.yml -f optional/docker-compose.traffic-portal-v2.yml -f optional/docker-compose.traffic-portal-v2-test.yml up --no-deps tpv2-e2e-test

(It's fine if it starts to start up as long as building the docker image succeeded, we're mainly only testing that the compose file can be used.)

Related PRs: #7938, #7936, #7925

If this is a bugfix, which Traffic Control versions contained the bug?

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[rimashah25]

LGTM