Fix build output

[michaeldwan]

Multiplexing stderr to a buffer accidentally broke TTY detection in docker when ran as a subprocess. This PR restores it by creating a pty/tty for the subprocess when necessary.

Also fixes:

• --progress was not being passed to the second build step.
• stops using a deprecated flag on the docker cli that was causing a warning

Before

$ go run ../../../../cmd/cog build -t cog-test
Building Docker image from environment in cog.yaml as cog-test...
#0 building with "desktop-linux" instance using docker driver

#1 [internal] load build definition from Dockerfile
#1 transferring dockerfile: 560B done
#1 DONE 0.0s

#2 [auth] docker/dockerfile:pull token for registry-1.docker.io
#2 DONE 0.0s
...

After

❯ go run ../../../../cmd/cog build -t cog-test
Building Docker image from environment in cog.yaml as cog-test...
[+] Building 0.5s (12/12) FINISHED                         docker:desktop-linux
 => [internal] load build definition from Dockerfile                       0.0s
 => => transferring dockerfile: 560B                                       0.0s
 => resolve image config for docker-image://docker.io/docker/dockerfile:1  0.1s
...