Drop relay-pkg.so from docker files

[tillkruss]

The pkg builds will be removed in Relay 1.x.

See cachewerk/relay-docs@9b2a636.
See cachewerk/linux-repos@ff6bf56 [RISKY].

Replaces #100.
Closes #56.

[szepeviktor]

@tillkruss Recent Docker builds:

0.592 The following packages have unmet dependencies:
0.634  php8.2-relay : Depends: libhiredis1.1.0 (>= 1.1.0) but it is not installable
0.634                 Depends: libck0 (>= 0.7.0) but it is not going to be installed

Even Debian 12 has only libhiredis0.14
https://packages.debian.org/source/bullseye/hiredis

libhiredis1.1.0 was release in Debian 13.

[tillkruss]

Looks like a lot of these need a custom install now: https://github.com/cachewerk/relay/actions/runs/19716040587