Bump Alpine to 3.21

[mm503]

Hi @mxpv ,

Alpine 3.17 went EOL on 2024-11-22 so I bumped it to the latest one (3.21).

Relevant changes:

• ffmpeg 5.1.4-r0 -> 6.1.2-r1
• python3 3.10.15-r0 -> 3.12.9-r0
• libc6-compat 1.2.3-r6 -> gcompat 1.1.0-r4 (the symlink in your Dockerfile is still valid)

I've beeb running this build along with my other change at home and it works well.

Log fragment:

│ time="2025-03-22T17:53:42Z" level=info msg="skipping due to max_age filter (223d > 14d)" episode_id=vtMy5l0rLx8 filter=max_age                                                                                                                                                  │
│ time="2025-03-22T17:53:42Z" level=info msg="skipping due to max_age filter (167d > 14d)" episode_id=xvDmR2n_D6E filter=max_age                                                                                                                                                  │
│ time="2025-03-22T17:53:42Z" level=info msg="skipping due to already downloaded" episode_id=y85OjRF5Ie8                                                                                                                                                                          │
│ time="2025-03-22T17:53:42Z" level=info msg="download count: 1"                                                                                                                                                                                                                  │
│ time="2025-03-22T17:53:42Z" level=info msg="! downloading episode https://youtube.com/watch?v=lDXxDA5uZvw" episode_id=lDXxDA5uZvw index=0                                                                                                                                       │
│ time="2025-03-22T17:53:53Z" level=info msg="creating file: /data/podsync/m-74gear/lDXxDA5uZvw.mp4" name=m-74gear/lDXxDA5uZvw.mp4                                                                                                                                                │
│ time="2025-03-22T17:53:54Z" level=info msg="successfully downloaded file \"lDXxDA5uZvw\"" episode_id=lDXxDA5uZvw index=0                                                                                                                                                        │
│ time="2025-03-22T17:53:54Z" level=info msg="downloaded 1 episode(s)"                                                                                                                                                                                                            │
│ time="2025-03-22T17:53:54Z" level=info msg="running cleaner" count=40 feed_id=m-74gear                                                                                                                                                                                          │
│ time="2025-03-22T17:53:54Z" level=info msg="creating file: /data/podsync/m-74gear.xml" name=m-74gear.xml