Environment Variable Interpolation is Order Dependent

[TechnoChimp]

What happened:
Variables are not interpolated when the variable assignment is ordered after the variable use.

What you expected to happen:
I would expect variable interpolation to succeed regardless of the order of the variables.

How to reproduce it (as minimally and precisely as possible):
Define variables for a container spec in the following order:

• name: var2
  value: $(var1).testString2
• name: var1
  value: testString1

Check the values of the variables inside the container and this is the result:
var1=testString1
var2='$(var1).testString2'

Anything else we need to know?:
Swapping the variable order resolves the issue.

Environment:

• Kubernetes version (use kubectl version):
  Client Version: version.Info{Major:"1", Minor:"18", GitVersion:"v1.18.0", GitCommit:"9e991415386e4cf155a24b1da15becaa390438d8", GitTreeState:"clean", BuildDate:"2020-03-25T14:58:59Z", GoVersion:"go1.13.8", Compiler:"gc", Platform:"darwin/amd64"}
  Server Version: version.Info{Major:"1", Minor:"15+", GitVersion:"v1.15.10-eks-bac369", GitCommit:"bac3690554985327ae4d13e42169e8b1c2f37226", GitTreeState:"clean", BuildDate:"2020-02-26T01:12:54Z", GoVersion:"go1.12.12", Compiler:"gc", Platform:"linux/amd64"}

• Cloud provider or hardware configuration:
  AWS EKS

• OS (e.g: cat /etc/os-release):
  NAME="Alpine Linux"
  ID=alpine
  VERSION_ID=3.9.4
  PRETTY_NAME="Alpine Linux v3.9"
  HOME_URL="https://alpinelinux.org/"
  BUG_REPORT_URL="https://bugs.alpinelinux.org/"

• Kernel (e.g. uname -a):
  Linux netshoot-6df4df959b-mwhkh 4.14.173-137.229.amzn2.x86_64 Unit test coverage in Kubelet is lousy. (~30%) #1 SMP Wed Apr 1 18:06:08 UTC 2020 x86_64 Linux

• Install tools:

• Network plugin and version (if this is a network-related bug):

• Others:

[TechnoChimp]

/sig apps

[liggitt]

since envvars can reference each other, and cycles are possible, this matches behavior of defining envvars in a shell, etc. this behavior should be documented.

[wawa0210]

/assign