The `-q` flag is not implemented by `docker buildx`, which results in an
output:
```
WARN[0000] quiet currently not implemented.
```
In the same way, the build output is not logged to `stdout` (but
`stderr`). This means we now dump the whole build process to a file and
if the `docker buildx` command fails, then we output those logs by
printing the contents of that file. This will also reduce the overall
verbosity and aligns to the original `docker build` behavior.
Signed-off-by: Sascha Grunert <mail@saschagrunert.de>
54db83eef8