[improvement sugestion] fail build fast for -dir (or create it)

[weakcamel]

Description

Would be great if linuxkit verified input parameters or handled them in a more gracious way.

Steps to reproduce the issue:

$ time linuxkit build -format vmdk -dir output platform-vmware.yml
Extract kernel image: docker.io/linuxkit/kernel:5.10.104
Add init containers:
Process init image: docker.io/linuxkit/init:14df799bb3b9e0eb0491da9fda7f32a108a2e2a5
Process init image: docker.io/linuxkit/runc:436357ce16dd663e24f595bcec26d5ae476c998e
Process init image: docker.io/linuxkit/containerd:eeb3aaf497c0b3f6c67f3a245d61ea5a568ca718
Process init image: docker.io/linuxkit/ca-certificates:4de36e93dc87f7ccebd20db616ed10d381911d32
Add onboot containers:
  Create OCI config for linuxkit/sysctl:e5959517fab7b44692ad63941eecf37486e73799
Add service containers:
  Create OCI config for linuxkit/getty:06f34bce0facea79161566d67345c3ea49965437
  Create OCI config for linuxkit/rngd:331294919ba6d953d261a2694019b659a98535a4
  Create OCI config for linuxkit/dhcpcd:2a8ed08fea442909ba10f950d458191ed3647115
  Create OCI config for nginx:1.13.8-alpine
Create outputs:
  output/platform-vmware.vmdk
FATA[0020] Error writing outputs: Error writing vmdk output: open output/platform-vmware.vmdk: no such file or directory
real	0m20.118s
user	0m26.201s
sys	0m2.044s

Describe the results you received:

When output dir doesn't exist, linuxkit works as usual and fails at the stage of writing out the output image.

Describe the results you expected:

It would be great if linuxkit did one of:

• fail immediately (not after downloading all the images and processing them) if output dir doesn't exist, or
• not fail and create the output directory automatically

At the very least it should document that in help message, i.e. say it has to be an existing directory:

$  linuxkit build -help
USAGE: linuxkit build [options] <file>[.yml] | -

Options:
[...]
  -cache value
    	Directory for caching and finding cached image, overrides env var LINUXKIT_CACHE (default /Users/waldekm/.linuxkit/cache)
[...]
  -dir string
    	Directory for output files, default current directory

Note that the -cache option silently creates the output dir.

Additional information you deem important (e.g. issue happens only occasionally):

[deitch]

I see no issues with an os.MkdirAll(dir) for the target directory. Open a PR @weakcamel ? Just need to it around here