DPC-3779: Set JEKYLL_ENV when building docker image

[kyeah]

🎫 Ticket

https://jira.cms.gov/browse/DPC-3779

🛠 Changes

• Set JEKYLL_ENV so it's used when building the docker image

ℹ️ Context for reviewers

The JEKYLL_ENV isn't read by docker (even it's set in the local environment), so jekyll.environment always evaluates to development.

✅ Acceptance Validation

Build image:

export JEKYLL_ENV=prod
docker-compose -f docker-compose.yml build static_site

Then started a container and checked the _site/index.html to verify that it's using the prod tag for Adobe Analytics.

Also deployed to stage and confirmed it's using the qa URL.

🔒 Security Implications

• This PR adds a new software dependency or dependencies.
• This PR modifies or invalidates one or more of our security controls.
• This PR stores or transmits data that was not stored or transmitted before.
• This PR requires additional review of its security implications for other reasons.

If any security implications apply, add Jason Ashbaugh (GitHub username: StewGoin) as a reviewer and do not merge this PR without his approval.