（问题解决篇）ubuntu更新时，出现错误E: Some index files failed to download。。。

在ubuntu更新时，出现错误E: Some index files failed to download.They have been ignored, or old ones used instead.
在sudo apt-get update的时候出现了下列问题：

root@0cc8c10d217a:~# sudo apt-get update
sudo: unable to resolve host 0cc8c10d217a
Err http://archive.ubuntu.com trusty InRelease

Err http://archive.ubuntu.com trusty-security InRelease

Err http://archive.ubuntu.com trusty-updates InRelease

Err http://archive.ubuntu.com trusty-proposed InRelease

Err http://archive.ubuntu.com trusty-backports InRelease

Err http://archive.ubuntu.com trusty Release.gpg
Could not resolve 'archive.ubuntu.com'
Err http://archive.ubuntu.com trusty-security Release.gpg
Could not resolve 'archive.ubuntu.com'
Err http://archive.ubuntu.com trusty-updates Release.gpg
Could not resolve 'archive.ubuntu.com'
Err http://archive.ubuntu.com trusty-proposed Release.gpg
Could not resolve 'archive.ubuntu.com'
Err http://archive.ubuntu.com trusty-backports Release.gpg
Could not resolve 'archive.ubuntu.com'
Reading package lists... Done
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty/InRelease
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-security/InRelease
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-updates/InRelease
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-proposed/InRelease
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-backports/InRelease
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty/Release.gpg Could not resolve 'archive.ubuntu.com'

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-security/Release.gpg Could not resolve 'archive.ubuntu.com'

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-updates/Release.gpg Could not resolve 'archive.ubuntu.com'

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-proposed/Release.gpg Could not resolve 'archive.ubuntu.com'

W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-backports/Release.gpg Could not resolve 'archive.ubuntu.com'

W: Some index files failed to download. They have been ignored, or old ones used instead.

所以在docker里面装nginx的时候也出现问题：
root@0cc8c10d217a:~# apt-get install nginx
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
nginx-common nginx-core
Suggested packages:
fcgiwrap nginx-doc
The following NEW packages will be installed:
nginx nginx-common nginx-core
0 upgraded, 3 newly installed, 0 to remove and 241 not upgraded.
Need to get 349 kB of archives.
After this operation, 1,300 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y
WARNING: The following packages cannot be authenticated!
nginx-common nginx-core nginx
Install these packages without verification? [y/N] Y
Err http://archive.ubuntu.com/ubuntu/ trusty-updates/main nginx-common all 1.4.6-1ubuntu3.7
Could not resolve 'archive.ubuntu.com'
Err http://archive.ubuntu.com/ubuntu/ trusty-updates/main nginx-core amd64 1.4.6-1ubuntu3.7
Could not resolve 'archive.ubuntu.com'
Err http://archive.ubuntu.com/ubuntu/ trusty-updates/main nginx all 1.4.6-1ubuntu3.7
Could not resolve 'archive.ubuntu.com'
E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/n/nginx/nginx-common_1.4.6-1ubuntu3.7_all.deb Could not resolve 'archive.ubuntu.com'

E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/n/nginx/nginx-core_1.4.6-1ubuntu3.7_amd64.deb Could not resolve 'archive.ubuntu.com'

E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/n/nginx/nginx_1.4.6-1ubuntu3.7_all.deb Could not resolve 'archive.ubuntu.com'

E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
因为刚刚换过apt-get源 ，所以百度了一下知道大概的做法是DNS出现了问题
应着重查看以下方面：
cat /etc/resolv.conf查看里面的domain, search和nameserver配置是否正确。
然后一看是DNS出现了问题，所以将nameserver修改成8.8.8.8后，问题得到了解决。