feat(lang): Embed dockerfile to build.envd

[gaocegege]

Description

def build():
    dockerfile(`
RUN apt instal

ENV PATH $PATH:a
`
)

---

Message from the maintainers:

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[aseaday]

Good idea, But we need more details about the limit in embeded Dockerfile.

[xieydd]

Good idea, it will be useful for some developer is familiar with dockerfile usage.

[kemingy]

refer to: https://github.com/moby/buildkit/blob/18879bc80c235d7eba6610dc94cf54d543e2328d/examples/dockerfile2llb/main.go#L12