Docker message: Automatically disabled Acquire::http::Pipeline-Depth due to incorrect response from server/proxy - apache

Question

Installing apache2 into a Ubuntu 16.04 image of Docker , I am getting the follow message
W: http://archive.ubuntu.com/ubuntu/pool/main/g/gdbm/libgdbm3_1.8.3-13.1_amd64.deb: Automatically disabled Acquire::http::Pipeline-Depth due to incorrect response from server/proxy. (man 5 apt.conf).
That is the Dockerfile:
FROM ubuntu:16.04
#RUN apt-get update
#https://github.com/phusion/baseimage-docker/issues/319
RUN apt-get update && apt-get install -y --no-install-recommends apt-utils
RUN apt-get install -y apache2
When I open the image I see the /var/www/html folder, meaning, the apache was installed.
What message is that? Is it an error or can I consider apache as fully installed?

Answer 1

Pipelining is a feature of the HTTP/1.1 protocol. From the RFC 7230 :
A client that supports persistent connections MAY "pipeline" its
requests (i.e., send multiple requests without waiting for each
response). A server MAY process a sequence of pipelined requests in
parallel if they all have safe methods (Section 4.2.1 of [RFC7231]),
but it MUST send the corresponding responses in the same order that
the requests were received.
This feature can be activated in apt with the setting Acquire::http::Pipeline-Depth. From man apt.conf:
The setting Acquire::http::Pipeline-Depth can be used to enable HTTP pipelining (RFC 2616 section 8.1.2.2) which can be beneficial e.g. on high-latency connections. It specifies how many requests are sent in a pipeline. APT tries to detect and workaround misbehaving webservers and proxies at runtime, but if you know that yours does not conform to the HTTP/1.1 specification pipelining can be disabled by setting the value to 0. It is enabled by default with the value 10.
The message you see means that your connection with the apt repository doesn't support pipelining, (probably because of some kind of proxy) and that this feature was automatically disabled by apt. The installation will maybe takes a bit more time but you can consider your apache server fully installed.