container source plugin supports watching update of a specified tag

[bianjp]

This PR extends the container source plugin to support watching update of a specified tag.

Previously the container parameter can only be image_path, now it will also accept image_path:tag. If no tag is given, the plugin's behavior is the same as before.

When watching for a specified tag, the tag's version is the update time of the tag. Single image tag itself does not have any update time, we take the creation time of the image ID referred by the tag. Theoretically speaking the creation time is not 100% reliable but in practice it is acceptable and more readable than image ID.

Sample config:

# watch the latest tag
[oraclelinux]
source = "container"
container = "library/oraclelinux"

# watch the update of oraclelinux:8 tag
[oraclelinux-8]
source = "container"
container = "library/oraclelinux:8"

Resolve #241

[yan12125]

Thanks! I'm not sure if it will be confusing to return different data types with different configurations, though.

[lilydjwg]

if it will be confusing to return different data types with different configurations, though.

It's already happening, e.g. for those git-hosting sites.

[yan12125]

Oh you're right. Indeed I was once confused by cases like use_latest_release and use_max_tag in the github source.