Show progress for docker commands

When building a crate for docs.rs, I get the following output, followed by 15 minutes without anything happening:

2019/10/25 01:20:53 [INFO] rustwide::cmd::sandbox: pulling image rustops/crates-build-env from Docker Hub
2019/10/25 01:20:53 [INFO] rustwide::cmd: running `"docker" "pull" "rustops/crates-build-env"`
2019/10/25 01:20:55 [INFO] rustwide::cmd: [stdout] Using default tag: latest
2019/10/25 01:20:56 [INFO] rustwide::cmd: [stdout] latest: Pulling from rustops/crates-build-env
2019/10/25 01:20:56 [INFO] rustwide::cmd: [stdout] 5667fdb72017: Already exists
2019/10/25 01:20:56 [INFO] rustwide::cmd: [stdout] d83811f270d5: Already exists
2019/10/25 01:20:56 [INFO] rustwide::cmd: [stdout] ee671aafb583: Already exists
2019/10/25 01:20:56 [INFO] rustwide::cmd: [stdout] 7fc152dfb3a6: Already exists
2019/10/25 01:20:56 [INFO] rustwide::cmd: [stdout] 5c0b1d519321: Pulling fs layer
2019/10/25 01:20:56 [INFO] rustwide::cmd: [stdout] 4f2b8f293757: Pulling fs layer
2019/10/25 01:20:56 [INFO] rustwide::cmd: [stdout] 2fa8e23f28c7: Pulling fs layer
2019/10/25 01:20:57 [INFO] rustwide::cmd: [stdout] 5c0b1d519321: Verifying Checksum
2019/10/25 01:20:57 [INFO] rustwide::cmd: [stdout] 5c0b1d519321: Download complete
2019/10/25 01:20:57 [INFO] rustwide::cmd: [stdout] 2fa8e23f28c7: Download complete
2019/10/25 01:20:57 [INFO] rustwide::cmd: [stdout] 5c0b1d519321: Pull complete

If I run docker pull rustops/crates-build-env manually on the host machine, I can see that it's trying to download a 1.3 GB image, which takes quite a while on my internet connection. Would it be possible to show this progress in rustwide so it doesn't seem like nothing is happening?

Start by tracing Docker command execution through the rustwide::cmd and rustwide::cmd::sandbox entry points, focusing on the docker pull output shown in the issue. Determine how progress can be surfaced while the image downloads, then verify that long-running pulls visibly report activity instead of appearing idle.