image(node): upgrade node:9.4-alpine to node:12-alpine

[jjangga0214]

This has some benefits.

1. Users will be able to use modern code without transpiling or configuring NODE_IMAGE.
2. As the new image is just 12(not specifying minor version), a new feature(minor version up) and security patch(patch version up) will be automatically applied in the future, without any breaking change.
3. This is LTS version. Therefore there will be long-term stable support and update, which makes sense for 12 to be a default version.
4. In the code level, 12 is compatible with 9.4(https://node.green/). Thus the majority of users who've been using the default version(9.4) do not have to manually configure NODE_IMAGE even with the new default version.

This might close #24, but I'm not sure if other languages' default versions are be good to be changed.

[mhausenblas]

Perfect, thank you @jjangga0214!