Reading Env did not work on v0.4.0

[niusmallnan]

Just take RancherOS as an example: https://github.com/rancher/os

Try v0.3.4:

$ ./.dapper -d -m bind -s
...
DEBU[0001] Reading Env: DAPPER_DOCKER_SOCKET=true
DEBU[0001] Reading Env: DAPPER_SOURCE=/go/src/github.com/rancher/os
...
DEBU[0001] Running shell in os:master
DEBU[0001] Exec /usr/bin/docker run [--rm -i --name os-awbwhED -t -v /var/run/docker.sock:/var/run/docker.sock -v /opt/official/os/.:/go/src/github.com/rancher/os/ -e DAPPER_UID=0 -e DAPPER_GID=0 -e VERSION -e DEV_BUILD -e RUNTEST -e DEBUG -e APTPROXY -e ENGINE_REGISTRY_MIRROR -e KERNEL_CHECK -e APPEND_SYSTEM_IMAGES --entrypoint /bin/bash -e TERM --privileged os:master -]

Try v0.4.0:

$ ./.dapper -d -m bind -s
...
DEBU[0005] Running shell in os:master
DEBU[0005] Exec /usr/bin/docker run [--rm -i --name os-NzKWvTZ -t -v /opt/official/os/.:/source/ -e DAPPER_UID=0 -e DAPPER_GID=0 --entrypoint /bin/bash -e TERM os:master -]

v0.3.4 can work well, but v0.4.0 can not. It seems that the latest version does not execute readEnv, this break the ROS CI.