Replace hard coded docker command to environment variable for support podman

[konono]

What changes were proposed in this pull request?

This PR is related to elyra-ai#3082
I found a problem where docker is hard coded in the validate-runtime-image method of the Makefile and podman cannot be used.
In a previous PR, it was implemented to switch between docker and podman using the CONTAINER_EXEC environment variable, so I followed that and fixed it.

How was this pull request tested?

❯ CONTAINER_EXEC=podman make validate-runtime-image image=localhost/test:v3
WARNING: You are using pip version 21.2.4; however, version 24.0 is available.
You should consider upgrading via the '/Applications/Xcode.app/Contents/Developer/usr/bin/python3 -m pip install --upgrade pip' command.
***********************************************************
Validating container image localhost/test:v3
-----------------------------------------------------------
=> Loading container image ...
=> Checking container image localhost/test:v3 for curl...
=> Checking container image localhost/test:v3 for python3...
=> Checking notebook execution...
=== SNIP ===
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   910  100   910    0     0   3137      0 --:--:-- --:--:-- --:--:--  3148
Input Notebook:  simple_execute.ipynb
Output Notebook: output.ipynb
Executing:   0%|          | 0/3 [00:00<?, ?cell/s]Executing notebook with kernel: python3
Executing DESC: 100%|██████████| 3/3 [00:00<00:00,  3.40cell/s]
-----------------------------------------------------------
=> Container image localhost/test:v3 is a suitable Elyra runtime image

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