Update docker README

[knqyf263]

If I run command according to README.md, vuls prepare will fail.
I think that it is because stdin is not opened.

$ docker run --rm -v ~/.ssh:/root/.ssh:ro -v $PWD:/vuls -v $PWD/vuls-log:/var/log/vuls vuls/vuls prepare -config=./config.toml
time="2017-01-21T13:07:34Z" level=info msg="Start Preparing (config: ./config.toml)"
time="2017-01-21T13:07:34Z" level=info msg="Validating Config..."
time="Jan 21 13:07:34" level=info msg="Detecting OS... "
time="Jan 21 13:07:34" level=info msg="Detecting OS of servers... "
time="Jan 21 13:07:35" level=info msg="(1/1) Detected: vagrant: centos 6.8"
time="Jan 21 13:07:35" level=info msg="Detecting OS of containers... "
time="Jan 21 13:07:35" level=info msg="Checking sudo configuration... "
time="Jan 21 13:07:36" level=info msg="sudo ... OK"
time="Jan 21 13:07:36" level=info msg="The following servers need dependencies installed"
time="Jan 21 13:07:36" level=info msg="  - yum-plugin-changelog on vagrant"
time="Jan 21 13:07:36" level=info msg="Is this ok to install dependencies on the servers? [y/N]"
time="Jan 21 13:07:36" level=error msg="Failed to prepare: EOF"

If I add -it option, I can press y interactively.

docker run --rm -it -v ~/.ssh:/root/.ssh:ro -v $PWD:/vuls -v $PWD/vuls-log:/var/log/vuls vuls/vuls prepare -config=./config.toml
INFO[0000] Start Preparing (config: ./config.toml)
INFO[0000] Validating Config...
[Jan 21 13:07:52]  INFO [localhost] Detecting OS...
[Jan 21 13:07:52]  INFO [localhost] Detecting OS of servers...
[Jan 21 13:07:53]  INFO [localhost] (1/1) Detected: vagrant: centos 6.8
[Jan 21 13:07:53]  INFO [localhost] Detecting OS of containers...
[Jan 21 13:07:53]  INFO [localhost] Checking sudo configuration...
[Jan 21 13:07:54]  INFO [vagrant] sudo ... OK
[Jan 21 13:07:54]  INFO [localhost] The following servers need dependencies installed
[Jan 21 13:07:54]  INFO [localhost]   - yum-plugin-changelog on vagrant
[Jan 21 13:07:54]  INFO [localhost] Is this ok to install dependencies on the servers? [y/N]
y
[Jan 21 13:08:20]  INFO [vagrant] Installed: yum-plugin-changelog
[Jan 21 13:08:20]  INFO [localhost] All dependencies were installed correctly

[kotakanbe]

Many Thanks!