Fixes #1449 - avoid shell tricks, brace expansion, GOFLAGS in docker …

[StevenACoffman]

For universal portability, the build_dist.sh script uses posix shell, which the build_docker.sh script adopted after testing. Brace expansion is not supported in the posix shell, and if more tags were ever added, was a brittle solution anyway.

I verified this works by running the script to build an image, and then running the latest built image:

./build_docker.sh
docker run -it $(docker images | awk '{print $3}' | awk 'NR==2') /usr/local/bin/tailscale version

and this produces the following output:

1.5.189
  tailscale commit: 937a46d12672c8afe127e43b7da2ea4222ffc4ec
  go version: go1.16

Signed-off-by: Steve Coffman steve@khanacademy.org

[bradfitz]

Can you git push -f a new single squashed commit in our existing style? (same as Go style: https://golang.org/wiki/CommitMessage)

Like:

build_docker.sh, Dockerfile: fix bug with shell quoting

Fixes #1449

[StevenACoffman]

Ah, I see, but leave the DCO. Sorry for the noise.

Another reason to avoid GOFLAGS here for now is because you cannot currently have multiple values for flags other than -tags without the last one "winning".

[StevenACoffman]

Also, not sure why the Windows / test fails, as this shouldn't affect it, so I'm guessing it's flaky or was introduced by something before this.