strange "apt-get install" behavior stumping me

docker-build.sh-session

$ docker build 'https://gist.github.com/5378a5d2c49abebebbc4.git' || echo >&2 'WAT??  WHY YOU FAIL???'
Sending build context to Docker daemon 48.13 kB
Sending build context to Docker daemon 
Step 0 : FROM debian:sid
 ---> 4b0273baed8f
Step 1 : RUN apt-get update && apt-get install -y ca-certificates --no-install-recommends
 ---> Running in bf5bd4bf6f86
Get:1 http://http.debian.net sid InRelease [206 kB]
Err http://http.debian.net sid/main amd64 Packages
  
Get:2 http://http.debian.net sid/main amd64 Packages [6817 kB]
Fetched 7023 kB in 15s (464 kB/s)
Reading package lists...
Reading package lists...
Building dependency tree...
The following extra packages will be installed:
  libssl1.0.0 openssl
The following NEW packages will be installed:
  ca-certificates libssl1.0.0 openssl
0 upgraded, 3 newly installed, 0 to remove and 20 not upgraded.
Need to get 1872 kB of archives.
After this operation, 4558 kB of additional disk space will be used.
Get:1 http://http.debian.net/debian/ sid/main libssl1.0.0 amd64 1.0.1h-3 [1011 kB]
Get:2 http://http.debian.net/debian/ sid/main openssl amd64 1.0.1h-3 [668 kB]
Get:3 http://http.debian.net/debian/ sid/main ca-certificates all 20140325 [193 kB]
debconf: delaying package configuration, since apt-utils is not installed
Fetched 1872 kB in 5s (348 kB/s)
Selecting previously unselected package libssl1.0.0:amd64.
(Reading database ... 6853 files and directories currently installed.)
Preparing to unpack .../libssl1.0.0_1.0.1h-3_amd64.deb ...
Unpacking libssl1.0.0:amd64 (1.0.1h-3) ...
Selecting previously unselected package openssl.
Preparing to unpack .../openssl_1.0.1h-3_amd64.deb ...
Unpacking openssl (1.0.1h-3) ...
Selecting previously unselected package ca-certificates.
Preparing to unpack .../ca-certificates_20140325_all.deb ...
Unpacking ca-certificates (20140325) ...
Setting up libssl1.0.0:amd64 (1.0.1h-3) ...
debconf: unable to initialize frontend: Dialog
debconf: (TERM is not set, so the dialog frontend is not usable.)
debconf: falling back to frontend: Readline
debconf: unable to initialize frontend: Readline
debconf: (Can't locate Term/ReadLine.pm in @INC (you may need to install the Term::ReadLine module) (@INC contains: /etc/perl /usr/local/lib/perl/5.18.2 /usr/local/share/perl/5.18.2 /usr/lib/perl5 /usr/share/perl5 /usr/lib/perl/5.18 /usr/share/perl/5.18 /usr/local/lib/site_perl .) at /usr/share/perl5/Debconf/FrontEnd/Readline.pm line 7.)
debconf: falling back to frontend: Teletype
Setting up openssl (1.0.1h-3) ...
Setting up ca-certificates (20140325) ...
debconf: unable to initialize frontend: Dialog
debconf: (TERM is not set, so the dialog frontend is not usable.)
debconf: falling back to frontend: Readline
debconf: unable to initialize frontend: Readline
debconf: (Can't locate Term/ReadLine.pm in @INC (you may need to install the Term::ReadLine module) (@INC contains: /etc/perl /usr/local/lib/perl/5.18.2 /usr/local/share/perl/5.18.2 /usr/lib/perl5 /usr/share/perl5 /usr/lib/perl/5.18 /usr/share/perl/5.18 /usr/local/lib/site_perl .) at /usr/share/perl5/Debconf/FrontEnd/Readline.pm line 7.)
debconf: falling back to frontend: Teletype
Processing triggers for libc-bin (2.19-4) ...
Processing triggers for ca-certificates (20140325) ...
Updating certificates in /etc/ssl/certs... 168 added, 0 removed; done.
Running hooks in /etc/ca-certificates/update.d....done.
WAT??  WHY YOU FAIL???

Dockerfile

FROM debian:sid
RUN apt-get update && apt-get install -y ca-certificates --no-install-recommends