Cannot append to /etc/hosts in a Dockerfile

[stapelberg]

This is my Dockerfile:

FROM tianon/debian:sid

RUN apt-get update && echo 2014-01-30
RUN apt-get dist-upgrade -y
RUN apt-get install -y python-pip python-dev python-psycopg2 python-flup lighttpd memcached

RUN mount
RUN cat /proc/mounts
RUN df -h
RUN echo "127.0.0.2 i3faq.localhost" >> /etc/hosts

When executing docker build ., I get:

Uploading context 30.72 kB
Uploading context 
Step 1 : FROM tianon/debian:sid
 ---> f61612c324ff
Step 2 : RUN apt-get update && echo 2014-01-30
 ---> Using cache
 ---> 1973f5c6e900
Step 3 : RUN apt-get dist-upgrade -y
 ---> Using cache
 ---> 1c05a139c269
Step 4 : RUN apt-get install -y python-pip python-dev python-psycopg2 python-flup lighttpd memcached
 ---> Using cache
 ---> 051b8c39726c
Step 5 : RUN mount
 ---> Running in 343718a814eb
none on / type aufs (rw,relatime,si=9c43d030a7b41b27)
/dev/disk/by-uuid/4c66e715-0276-4ac6-a6e0-2ba8d072134a on /.dockerinit type ext4 (ro,relatime,errors=remount-ro,data=ordered)
/dev/disk/by-uuid/4c66e715-0276-4ac6-a6e0-2ba8d072134a on /.dockerenv type ext4 (ro,relatime,errors=remount-ro,data=ordered)
/dev/disk/by-uuid/4c66e715-0276-4ac6-a6e0-2ba8d072134a on /etc/resolv.conf type ext4 (ro,relatime,errors=remount-ro,data=ordered)
/dev/disk/by-uuid/4c66e715-0276-4ac6-a6e0-2ba8d072134a on /etc/hostname type ext4 (ro,relatime,errors=remount-ro,data=ordered)
/dev/disk/by-uuid/4c66e715-0276-4ac6-a6e0-2ba8d072134a on /etc/hosts type ext4 (ro,relatime,errors=remount-ro,data=ordered)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
shm on /dev/shm type tmpfs (rw,nosuid,nodev,noexec,relatime,size=65536k)
devpts on /dev/tty1 type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
devpts on /dev/pts type devpts (rw,relatime,mode=600,ptmxmode=666)
devpts on /dev/ptmx type devpts (rw,relatime,mode=600,ptmxmode=666)
 ---> f974db057292
Step 6 : RUN cat /proc/mounts
 ---> Running in a3e400d9f5a5
rootfs / rootfs rw 0 0
none / aufs rw,relatime,si=9c43d037f3492b27 0 0
/dev/disk/by-uuid/4c66e715-0276-4ac6-a6e0-2ba8d072134a /.dockerinit ext4 ro,relatime,errors=remount-ro,data=ordered 0 0
/dev/disk/by-uuid/4c66e715-0276-4ac6-a6e0-2ba8d072134a /.dockerenv ext4 ro,relatime,errors=remount-ro,data=ordered 0 0
/dev/disk/by-uuid/4c66e715-0276-4ac6-a6e0-2ba8d072134a /etc/resolv.conf ext4 ro,relatime,errors=remount-ro,data=ordered 0 0
/dev/disk/by-uuid/4c66e715-0276-4ac6-a6e0-2ba8d072134a /etc/hostname ext4 ro,relatime,errors=remount-ro,data=ordered 0 0
/dev/disk/by-uuid/4c66e715-0276-4ac6-a6e0-2ba8d072134a /etc/hosts ext4 ro,relatime,errors=remount-ro,data=ordered 0 0
proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0
sysfs /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0
shm /dev/shm tmpfs rw,nosuid,nodev,noexec,relatime,size=65536k 0 0
devpts /dev/tty1 devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
devpts /dev/pts devpts rw,relatime,mode=600,ptmxmode=666 0 0
devpts /dev/ptmx devpts rw,relatime,mode=600,ptmxmode=666 0 0
 ---> 53a9e39f461f
Step 7 : RUN df -h
 ---> Running in 8a6f4c42ff1d
df: cannot read table of mounted file systems: No such file or directory
2014/01/30 21:36:57 The command [/bin/sh -c df -h] returned a non-zero code: 1

I think /etc/hosts should not be bind-mounted read-only by docker :).

[tianon]

This is a dup of #2267, but #2335 and #3357 are also interesting to note.

If you just need a quick workaround for changing the current hostname, you can use docker run -h i3faq.localhost .... It's very much less than ideal, and doesn't work for multiple hostnames (obviously), but it can do the job in a real pinch. Another option would be a custom DNS server for the container, but even that's just another very hacky workaround.

As noted on #2267, it is in the plans to stop having /etc/hosts be a bind mount. :)

Also, just as a side note (which is completely orthogonal to the issue at hand and is just an extra observation), that build log isn't failing on the /etc/hosts line, it's failing on the df line because /etc/mtab doesn't exist. :)