fix(docker): bundle zstd/lz4/xz runtime libs for pg_dump and mongodump

Alpine's postgresql-client and mongodb-tools dynamically link against
libzstd.so.1 (and libraries like lz4/xz that they may pull in for
compressed dumps); without those packages the binaries crash at
startup with 'Error loading shared library libzstd.so.1'. Bundle all
three so every supported driver loads cleanly.

apps/agent/Dockerfile

@@ -78,6 +78,9 @@ RUN apk add --no-cache \
       openssl \
       sqlite-libs \
       readline \
+      zstd-libs \
+      lz4-libs \
+      xz-libs \
     && addgroup -S backupy -g 1000 \
     && adduser  -S backupy -G backupy -u 1000