Unable to build in the ubuntu-xenial docker image: clap 2.28 requires Rust 1.20

/usr/bin/cargo build
    Updating registry `https://github.com/rust-lang/crates.io-index`
   Compiling bitflags v1.0.1
   Compiling integer-encoding v1.0.4
   Compiling num-traits v0.1.40
   Compiling byteorder v1.1.0
error: expected ident, found #
   --> /root/.cargo/registry/src/github.com-1ecc6299db9ec823/bitflags-1.0.1/src/lib.rs:423:29
    |
423 |                             #[allow(deprecated)]
    |                             ^
    | 
   ::: /root/.cargo/registry/src/github.com-1ecc6299db9ec823/bitflags-1.0.1/src/example_generated.rs
    |
4   | bitflags! {
    | - in this macro invocation

error: Could not compile `bitflags`.
Build failed, waiting for other jobs to finish...
error: build failed
Makefile:634: recipe for target 'check' failed
make[1]: *** [check] Error 101