Update preinstall script to update agent config from V2 to V3

Author: dhurley

test/docker/nginx-official-image/apk/Dockerfile

@@ -12,6 +12,7 @@ WORKDIR /agent
 COPY ./build/${PACKAGE_NAME}.apk /agent/build/${PACKAGE_NAME}.apk
 COPY ./ /agent
 
+RUN apk del nginx-agent \
 RUN apk add --allow-untrusted /agent/build/${PACKAGE_NAME}.apk
 
 RUN unlink /var/log/nginx/access.log

test/docker/nginx-official-image/deb/Dockerfile

@@ -14,6 +14,7 @@ COPY ./build/${PACKAGE_NAME}.deb /agent/build/${PACKAGE_NAME}.deb
 COPY ./ /agent
 
 RUN  apt-get update \ 
+    && apt purge -y nginx-agent \
     && apt install --no-install-recommends --no-install-suggests -y /agent/build/${PACKAGE_NAME}.deb \ 
     &&  rm /agent/build/${PACKAGE_NAME}.deb