docker images command yields timestamps as strings that cannot be formatted

The docker images command passes timestamps to the --format template as strings, rather than time/date types. This means there is no way to make it print the .CreatedAt field in any format other than the default, which looks like this:

2020-09-16 20:07:21 +0200 CEST

This is inflexible and makes machine parsing of the printed output needlessly difficult.

I used this command to verify that the template is receiving a string instead of a structured type:

$ docker images --format '{{printf "%T" .CreatedAt}}'
string

This limits the utility of the --format option. If the timestamp was instead passed to the template as a structured variable, practically any format could be printed, which would make it useful for either human reading or machine parsing.

The .CreatedSince field also suffers from this problem, always yielding a string like 11 days ago. Again, it would be easier to work with if it were a native type that could be formatted according to the user's needs.

Output of docker version:

Client:
 Version:           19.03.13
 API version:       1.40
 Go version:        go1.15.2
 Git commit:        4484c46
 Built:             Tue, 22 Sep 2020 16:21:48 +0700
 OS/Arch:           linux/arm64
 Experimental:      false

Server:
 Engine:
  Version:          19.03.13
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.15.2
  Git commit:       4484c46
  Built:            Tue Sep 22 09:21:48 2020
  OS/Arch:          linux/arm64
  Experimental:     true
 containerd:
  Version:          19.03.13
  GitCommit:        481103c8793316c118d9f795cde18060847c370e
 runc:
  Version:          1.0.0~rc92+dfsg1
  GitCommit:        1.0.0~rc92+dfsg1-5
 docker-init:
  Version:          0.18.0
  GitCommit:        

Start by tracing how docker images --format supplies .CreatedAt and .CreatedSince to Go templates. Verify the current behavior with the provided printf "%T" command, then identify the relevant CLI tests or formatting entry point. Done means both fields can be formatted as structured time values while preserving the intended output behavior.