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>

packages/podman-compose/slack-desc

@@ -0,0 +1,19 @@
+# HOW TO EDIT THIS FILE:
+# The "handy ruler" below makes it easier to edit a package description.
+# Line up the first '|' above the ':' following the base package name, and
+# the '|' on the right side marks the last column you can put a character in.
+# You must make exactly 11 lines for the formatting to be correct. It's also
+# customary to leave one space after the ':' except on otherwise blank lines.
+
+        |-----handy-ruler------------------------------------------------|
+podman-compose: podman-compose (external Compose provider for podman compose)
+podman-compose:
+podman-compose: A Python script that implements Docker Compose file support
+podman-compose: on top of podman, installed to /usr/local/bin so
+podman-compose: `podman compose` finds it as its external provider.
+podman-compose: Required by the WebUI's Compose panel. Bundled with its
+podman-compose: two runtime dependencies (PyYAML, python-dotenv) as plain
+podman-compose: pure-Python source.
+podman-compose:
+podman-compose: Homepage: https://github.com/containers/podman-compose
+podman-compose: