Replace vendored docker-compose with podman-compose

podman-compose and docker-compose aren't discovered the same way by
`podman compose` - verified live (a fake-binary test reading podman's own
provider-search error output) that docker-compose is searched for by
exact path across a fixed list of CLI-plugin directories, while
podman-compose is instead looked up as a plain command on $PATH. This
package installs to /usr/local/bin/podman-compose accordingly, not under
any cli-plugins/ directory.

Unlike docker-compose (a single static Go binary), podman-compose is a
Python script with two runtime dependencies neither of which ship with
Unraid's own Python3 - PyYAML and python-dotenv, vendored here as plain
pure-Python source (no C extension build; PyYAML's own fallback handles
its optional C accelerator being absent).

Verified end-to-end on a real host: with the previous docker-compose
binary temporarily moved aside to confirm podman-compose was actually the
one invoked, `podman compose up/ps/down` ran a real compose project
correctly, including a live HTTP check against the started service.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.github/workflows/build-packages.yml

@@ -2,7 +2,7 @@ name: Build Packages
 
 # Builds the eleven Slackware .txz packages defined under packages/
 # (podman, conmon, crun, netavark, aardvark-dns, passt, fuse-overlayfs,
-# catatonit, nftables, docker-compose, unraid-podman) inside a Slackware
+# catatonit, nftables, podman-compose, unraid-podman) inside a Slackware
 # container, verifies + consolidates their checksums, and uploads the
 # result as a workflow artifact.
 #