Create Android toolchain at Docker build time other than runtime

[wangkuiyi]

The Dockerfile.android file runs build_android.sh in the CMD directive. It seems that this would make it build Android toolchain every time we run docker run. If so, it seems more reasonable to build the toolchain at Docker build time.

[Xreki]

In fact, NDK is the official toolchain of Google. It contains all tools for all ANDROID_API and ANDROID_ABI. Standalone toolchain is a subset of NDK, and it is generated for specific ANDROID_ABI and ANDROID_API. The NDK is packaged into docker images, see.
Currently, Paddle can only be cross-compiled for Android using standalone toolchain. But for different arguments ANDROID_ABI and ANDROID_API, different version of standalone toolchain is needed to be generated. The main reason to build standalone toolchain every time when we run docker run, is to support all user given ANDROID_ABI and ANDROID_API.

[wangkuiyi]

Got it. I noticed that the building of standalone toolchain doesn't cost much time -- around 1 min. So it is reasonable to keep it in the docker-run stage.