Implement WithImagePullPolicy

[BenasB]

Trying to implement WithImagePullPolicy mentioned in #601. I tried to keep similar functionality to the Java counterpart where you can write a custom policy depending on the information about the cached image (e. g. policy that pulls the image if the currently cached image's Created property is 3 days old). Of course there's still the presets PullPolicy.Never, PullPolicy.Missing (default) and PullPolicy.Always like with docker run argument --pull.

I am not sure how I should proceed with tests – ideally I would want to for example test if PullPolicy.Never is working, so I would need to somehow delete the image if it's cached/pulled, but I am not sure how to do that, since I can't access the Docker.DotNet client underneath. Any suggestions appreciated.

How is this looking @HofmeisterAn ?

[HofmeisterAn]

👏 thanks for your contribution. PR looks good. I have just one question below.

I am not sure how I should proceed with tests – ideally I would want to for example test if PullPolicy.Never is working

You can try to pull an image we do not use, like alpine:edge. StartAsync will thrown an exception that it could not find the image.

[BenasB]

Thanks for the suggestion, yes using an image we don't use would make sense for PullPolicy.Never, but not sure how to handle PullPolicy.Always - it's useful in scenarios when an image with tag latest is cached on the docker client, but there's a new version of the latest tag in the registry and you want to always check if your cached version is up to date with the version in the registry.

[HofmeisterAn]

Thanks for the suggestion, yes using an image we don't use would make sense for PullPolicy.Never, but not sure how to handle PullPolicy.Always - it's useful in scenarios when an image with tag latest is cached on the docker client, but there's a new version of the latest tag in the registry and you want to always check if your cached version is up to date with the version in the registry.

Lets skip that for now. If CreateAsync overrides existing images we are good. I do not think PullPolicy.Always is easy to test.

[BenasB]

Sorry, I only had access to my corporate PC where I can't pull public GitHub repos, so I ended up winging it in the online vscode editor 😅 Finally got the chance to fix it, should be okay now. Excuse my commit history, it's a bit all over the place.

[HofmeisterAn]

Excuse my commit history, it's a bit all over the place.

No worries, we squash it anyway. Thanks again for your contribution.

[HofmeisterAn]

c48d858, closes #601.