Running docker_build fails if file is at root

[jschaf]

Description of the problem / feature request / question:

Trying to bazel run a docker_build.

If possible, provide a minimal example to reproduce the problem:

Minimal repo: https://gist.github.com/jschaf/5fbdc65730fe9cdf7dcf5ddd9c95e0da

bazel run //:hello

Environment info

• Operating System:

uname -a
Linux 4.10.9-1-ARCH #1 SMP PREEMPT Sat Apr 8 12:39:59 CEST 2017 x86_64 GNU/Linux

• Bazel version (output of bazel info release):

release 0.4.5- (@Non-Git)

• If bazel info release returns "development version" or "(@Non-Git)", please tell us what source tree you compiled Bazel from; git commit hash is appreciated (git rev-parse HEAD):

From Arch Linux AUR, it's downloading 4.5 from the github releases.

Have you found anything relevant by searching the web?

Nothing found.

Anything else, information or logs or outputs that would be helpful?

INFO: Found 1 target...
Target //:hello up-to-date:
  bazel-bin/hello-layer.tar
INFO: Elapsed time: 0.052s, Critical Path: 0.00s

INFO: Running command line: bazel-bin/hello
Skipping 24c4a0933bbd2f6c65ed283d84502fed4d22c77bacd109353929fea482da6025, already loaded.
Tagging 24c4a0933bbd2f6c65ed283d84502fed4d22c77bacd109353929fea482da6025 as bazel/:hello
Error parsing reference: "bazel/:hello" is not a valid repository/tag: invalid reference format
ERROR: Non-zero return code '1' from command: Process exited with status 1.

[aehlig]

The reference "bazel/:hello" looks suspiciously like wrong string operations; however, I'm not an expert in this area. Assigning to Damien for further triage.

[damienmg]

Dupe of #616