Add exec permission to controller in docker image

[liuyuan10]

metallb.yaml runs controller as non-root user but in docker image,
controller binary is owned by root. Because docker ADD keeps file
permission, this change makes sure that /controller can be exeucted by
non-root user in the docker image.

fixes #643

Thanks for sending a pull request! A few things before we get started:

1. If this is your first time, please read the contributing guide
2. For non-trivial pull requests, please file an
   issue first, and get
   agreement that the change is a good idea, and a general guideline
   for how it should be implemented, before sending code. Large PRs
   that weren't first discussed and agreed upon in an issue won't be
   accepted.
3. If the PR fixes a particular bug, please include the words "Fixed
   #" in the PR text, so that the bug auto-closes when
   the PR is merged.

[rata]

I'd like to understand under which setups this is needed, to see if other things might need fixing too, but seems fine to add this line if it solves a problem for someone :-)

@liuyuan10 heh, sorry, just realize I asked the same in the issue 😂. Please share, but only in one place is fine :)

cc @johananl @russellb @daxmc99 WDYT? :)

[daxmc99]

I agree this could be fixed by user permissions but anything that slows down contributions from users should be fixed.

LGTM

[rata]

LGTM. I'd add a comment mentioning umask, though, so we don't forget why we changed it in the first place :)

[rata]

Added a suggestion for the comment, please squash it. Otherwise, LGTM :)

[rata]

If you don't mind, I'll amend that simple comment myself and merge the ammended version :)

[rata]

LGTM. Closing the PR as I merged this locally and pushed.

As I mentioned, I amended the commit. For transparency, modified the commit message with what I changed.

You can find the commit here. Basically did the same for the speaker Dockerfile and changed the commit to reflect that (mentioned only the controller) and the comment I suggested changing here.

Thanks again for the PR!