Fails to build in dockerhub

Created by: yookoala

Looks like Dockerhub provides no way inject environment variable in the build process. So the ARG based docker image FROM statement fails: https://hub.docker.com/r/cangjians/pycangjie/builds/bklokftnqnqe4en7tgnzw6h/

It is, however, possible to build images in Travis then push to dockerhub, see "Pushing a Docker Image to a Registry" in their documentaiton:

Pushing a Docker Image to a Registry

In order to push an image to a registry, one must first authenticate via docker login. The email, username, and password used for login should be stored in the repository settings environment variables, which may be set up through the web or locally via the Travis CLI, e.g.:

travis env set DOCKER_USERNAME myusername
travis env set DOCKER_PASSWORD secretsecret

Within your .travis.yml prior to attempting a docker push or perhaps before docker pull of a private image, e.g.:

docker login -u="$DOCKER_USERNAME" -p="$DOCKER_PASSWORD"