making modifications based on shellcheck -S error

[elreydetoda]

NOTE: I didn't test these changes on my machine because I didn't have something to run them against. So, before merging you might want to check and make sure everything still work. ( in theory it should )

Description of changes: I modified the following scripts:

• codebuild/detect-images.sh ( line 42 )
• ecs/detect-images.sh ( line 70 )

so that they no longer used the [@] inside of if conditions, because based on a really cool tool called shellcheck it might cause errors: https://github.com/koalaman/shellcheck/wiki/SC2199

This was the command that I ran, with it's output:

$ shellcheck -S error */*.sh                       

In codebuild/detect-images.sh line 42:
    if [[ ! -z "${dhimages[@]+"${dhimages[@]}"}" ]]; then
               ^-- SC2199: Arrays implicitly concatenate in [[ ]]. Use a loop (or explicit * instead of @).

In ecs/detect-images.sh line 70:
    if [[ ! -z "${dhimages[@]+"${dhimages[@]}"}" ]]; then
               ^-- SC2199: Arrays implicitly concatenate in [[ ]]. Use a loop (or explicit * instead of @).

I only did the error severity of shellcheck for this PR, so that it was focused and it could be a way to start the conversation about using it in the repo.

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