Delete source code in the docker image

[duanqn]

/usr/local/src takes up several GBs in the resulting docker image. Deleting the source code can greatly reduce the size of the docker image.
If you are concerned about open source license compliance, you can put in a file containing URLs pointing to their original repositories.

[xkszltl]

This is not only for license compliance, but mostly for our own convenience during debugging.
If for licensing I may zip those code.

Because of the following, our version (including git hash) cannot point to where the code use in build come from:

1. Libs constantly requires patching, and those patch commit won't be available in repo.
2. We cannot couple our release cycle to upstreams, and it's not even possible, as our libs set growth, to align to all upstream release cycle. We often need to take things (both features and patches) from master branch.
3. Some libs don't even have a release cycle.

Initially I only keep source code itself, but later on due to user asks, I add history as well.

Even though several GB sounds like a lot by itself, it's only a few percent of the entire docker.
Given the convenience of having code in hand, I don't think it's a good deal to trade it for a few GB.

[xkszltl]

Note that these GB I mentioned are for storage.
Network traffic GB is another story, as they're expensive.
However, docker image is zipped.
For source code, they should have high compression ratio in docker image.