Update docker images to share scripts when possible

[malbarbo]

https://github.com/rust-lang/rust/pull/41958 added the possibility of sharing scripts used in docker images. This allows, for example, to use the same script to install sccache in all images, consequently updating sccache can be done updating only one file.

We should update all images to share scripts when possible.

To update an image, one can follow these sequence of steps:

• Choose an unchecked image in the below list;
• Replace long RUN lines in the dockerfile of the image with scripts (like sccache.sh and dum-init.sh in src/ci/docker/scripts/, see for example the updates of arm-android and dist-android). You can organize the dockerfile in sections to make it easy to follow. sccache should be installed last. This avoids the whole container rebuild when sscache is updated;
• See if files in COPY lines are duplicated in other images (duplicated files can be found using the command fdupes -r src/ci/). If there are duplicated files, move then to src/ci/docker/scripts/ and update the docker files;
• Test with the command src/ci/docker/run.sh image with the modified image still works (docker is required).

List of images (unchecked images need to be updated):

• [x] arm-android
• [ ] armhf-gnu
• [x] cross
• [ ] dist-aarch64-linux
• [x] dist-android
• [ ] dist-armhf-linux
• [ ] dist-arm-linux
• [ ] dist-armv7-linux
• [ ] dist-fuchsia
• [ ] dist-i586-gnu-i686-musl
• [ ] dist-i686-freebsd
• [ ] dist-i686-linux
• [ ] dist-mips64el-linux
• [ ] dist-mips64-linux
• [ ] dist-mipsel-linux
• [ ] dist-mips-linux
• [ ] dist-powerpc64le-linux
• [ ] dist-powerpc64-linux
• [ ] dist-powerpc-linux
• [ ] dist-s390x-linux
• [ ] dist-x86_64-freebsd
• [ ] dist-x86_64-linux
• [ ] dist-x86_64-musl
• [ ] dist-x86_64-netbsd
• [ ] emscripten
• [ ] i686-gnu
• [ ] i686-gnu-nopt
• [ ] x86_64-gnu
• [ ] x86_64-gnu-aux
• [ ] x86_64-gnu-debug
• [ ] x86_64-gnu-distcheck
• [ ] x86_64-gnu-full-bootstrap
• [ ] x86_64-gnu-incremental
• [ ] x86_64-gnu-llvm-3.7
• [ ] x86_64-gnu-nopt

[malbarbo]

cc @alexcrichton

[alexcrichton]

Thanks for writing this up @malbarbo! If you're interested, want to write out some detailed steps for how to accomplish this? If we've got that listed we could likely list this as E-easy which would probably help get the work items here picked off very quickly!

[malbarbo]

@alexcrichton I wrote the steps. What do you think? Is it enough?

[alexcrichton]

Looks great! If anyone's got any follow-up questions please feel free to ask here!

@malbarbo I think there were some examples of this in a previous PR of yours as well, mind linking those here?

[malbarbo]

@alexcrichton I linked the android dockerfile updates.

[alexcrichton]

Looks like @venkatagiri has taken care of the cross image!