Allow custom ports for readiness check, bump unrar to 6.8.2, add transmission-extra

[thespad]

---

• [x] I have read the contributing guideline and understand that I have made the correct modifications

---

Description:

Closes #250

Won't build until Alpine fix their arm building issue.

Benefits of this PR and context:

How Has This Been Tested?

Source / References:

[LinuxServer-CI]

I am a bot, here are the test results for this PR: https://ci-tests.linuxserver.io/lspipepr/transmission/4.0.3-r3-pkg-7be80a18-dev-69d8f8ced907e82c914d326bf6ee0a7baca19be0-pr-251/index.html https://ci-tests.linuxserver.io/lspipepr/transmission/4.0.3-r3-pkg-7be80a18-dev-69d8f8ced907e82c914d326bf6ee0a7baca19be0-pr-251/shellcheck-result.xml	Tag	Passed
amd64-4.0.3-r3-pkg-7be80a18-dev-69d8f8ced907e82c914d326bf6ee0a7baca19be0-pr-251	✅
arm64v8-4.0.3-r3-pkg-7be80a18-dev-69d8f8ced907e82c914d326bf6ee0a7baca19be0-pr-251	✅