JDK8: External.sanity: Docker file fails to build due to FUNCTIONAL_TEST_TAG error

**Summary**
Sanity.external docker file appears to require a syntax fix.

**Error message**
```
21:30:56 [exec] Step 22/27 : RUN git checkout FUNCTIONAL_TEST_TAG
21:30:56 [exec] ---> Running in 173e36e3275e
21:30:57 [exec] error: pathspec 'FUNCTIONAL_TEST_TAG' did not match any file(s) known to git
21:30:57 [exec] The command '/bin/sh -c git checkout FUNCTIONAL_TEST_TAG' returned a non-zero code: 1
21:30:57 [exec] 
```

**URL**
https://ci.adoptium.net/job/Test_openjdk8_hs_sanity.external_x86-64_linux_testList_0/1/console

**Next steps**
This should be assigned to the POC for external testing so they can fix it. Will add a comment in Slack.

Start with the Sanity.external Docker file and inspect the step that runs `git checkout FUNCTIONAL_TEST_TAG`; compare it with the failing CI console output. Verify the Docker build completes successfully and the external sanity test job no longer fails on the FUNCTIONAL_TEST_TAG command.

Written by the indexing model from the issue text.