Remove docker flags that can cause issues

[aidanhs]

See rust-lang/rust#39036

r? @alexcrichton

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

@bors: r+

[bors]

📌 Commit fef01d6 has been approved by alexcrichton

[bors]

⌛ Testing commit fef01d6 with merge 30d665a...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 30d665a to master...