fix(docker): bundle readline runtime so sqlite3 client loads

The Alpine sqlite3 binary dynamically links against libreadline.so.8;
without it the agent crashes at the validate stage of the pipeline
with 'Error loading shared library libreadline.so.8: No such file or
directory'.

Add the readline package to the runtime stage so sqlite3 starts
cleanly.

apps/agent/Dockerfile

@@ -77,6 +77,7 @@ RUN apk add --no-cache \
       mariadb-connector-c \
       openssl \
       sqlite-libs \
+      readline \
     && addgroup -S backupy -g 1000 \
     && adduser  -S backupy -G backupy -u 1000