Run the docker login command more safely (#160)
Run the docker login command more safely (#160)

Previously tracing mode meant that the Docker credentials were output
to stderr during the image publishing step. Travis strips secrets set
via Travis env vars from the logs (replacing the text with `[secure]`),
so these weren't visible, but it's safer to just not output them at all.

The command is now run in a sub-shell (saves having to re-enable tracing
after), with tracing disabled.

In addition, Docker recommends using `--password-stdin` over `--password`
since the former doesn't result in the credentials being added to bash
history (not that this is really relevant in CI, but it stops the warning).

Travis logs before:

```
+bin/build.sh heroku-18 heroku/heroku:18.nightly heroku/heroku:18-build.nightly
...
+docker login -u [secure] -p [secure]
WARNING! Using --password via the CLI is insecure. Use --password-stdin.
Login Succeeded
+docker push heroku/heroku:18.nightly
```

Travis logs after:

```
+bin/build.sh heroku-18 heroku/heroku:18.nightly heroku/heroku:18-build.nightly
...
+ set +x
Login Succeeded
+docker push heroku/heroku:18.nightly
```

Refs W-7496420.
heroku/stack-images