Replace ffmpeg4 with ffmpeg in Dockerfile

[LiBe-net]

see tvheadend/tvheadend@ee95c5e

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

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[LinuxServer-CI]

I am a bot, here are the test results for this PR:
https://ci-tests.linuxserver.io/lspipepr/tvheadend/23e2089e-pkg-a9f01cc1-dev-8e6f640b05185c50edaac2eecfbaac6eb8437efe-pr-274/index.html
https://ci-tests.linuxserver.io/lspipepr/tvheadend/23e2089e-pkg-a9f01cc1-dev-8e6f640b05185c50edaac2eecfbaac6eb8437efe-pr-274/shellcheck-result.xml

Tag	Passed
amd64-23e2089e-pkg-a9f01cc1-dev-8e6f640b05185c50edaac2eecfbaac6eb8437efe-pr-274	✅
arm64v8-23e2089e-pkg-a9f01cc1-dev-8e6f640b05185c50edaac2eecfbaac6eb8437efe-pr-274	✅

[LinuxServer-CI]

This pull request has been automatically marked as stale because it has not had recent activity. This might be due to missing feedback from OP. It will be closed if no further activity occurs. Thank you for your contributions.

[LiBe-net]

any human here?

[j0nnymoe]

Apologies, this container is long due a refresh (as I've mentioned in other threads) which will likely mean a change from alpine to ubuntu too for improved hardware transcoding options. Unfortunately free time to look at it is sparse currently but it will get looked at some point.

[aptalca]

We're also waiting on the status of an upstream PR: tvheadend/tvheadend#1966