Make sudo make docker-test work, too.

[filmil]

From @kpozin

$ sudo make docker-test
mkdir -p /tmp/rust_icu-root-target
docker run --tty --interactive \
                --user=0:0 \
                --volume=:/src/rust_icu \
                --volume=/tmp/rust_icu-root-target:/build/cargo \
                --volume=/root/.cargo:/usr/local/cargo \
                --env="CARGO_TEST_ARGS=" \
                filipfilmar/rust_icu_testenv-64:0.0.4
+ env
ICU_SOURCE_DIR=/src/icu
HOSTNAME=0b7d0ac807af
CARGO_TEST_ARGS=
RUST_ICU_SOURCE_DIR=/src/rust_icu
PWD=/src
ICU4C_BUILD_DIR=/build/icu4c-build
HOME=/root
CARGO_HOME=/usr/local/cargo
TERM=xterm
RUSTUP_HOME=/usr/local/rustup
SHLVL=1
RUST_VERSION=1.40.0
PATH=/usr/local/cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
CARGO_BUILD_DIR=/build/cargo
_=/usr/bin/env
+ cd /src/rust_icu
+ cargo install bindgen rustfmt
/entrypoint.sh: line 5: cargo: command not found
+ cd rust_icu_sys
/entrypoint.sh: line 7: cd: rust_icu_sys: No such file or directory
+ env LD_LIBRARY_PATH=/usr/local/lib cargo test
env: 'cargo': No such file or directory
+ cd rust_icu_common
/entrypoint.sh: line 11: cd: rust_icu_common: No such file or directory
+ env LD_LIBRARY_PATH=/usr/local/lib cargo test
env: 'cargo': No such file or directory
+ env LD_LIBRARY_PATH=/usr/local/lib cargo test
env: 'cargo': No such file or directory
make: *** [Makefile:38: docker-test] Error 127

[filmil]

@kpozin sorry about that. The PR above should fix the issue.

Tested with both sudo make docker-test and make docker-test and they both seem to work.