use destImageURL and minor nit fixes

[ajayk]

Failure to push image

➜  certko git:(main) ✗ ./certko --image-url=cgr.dev/chainguard/wolfi-base --ca-certs-file mycert.pem --dest-image-url docker.io/ajaykemparaj/wolfi-base:with-certs-test-2

2023/04/16 11:43:05 Failed to push modified image cgr.dev/chainguard/wolfi-base:withcerts: GET https://cgr.dev/token?scope=repository%3Achainguard%2Fwolfi-base%3Apush%2Cpull&service=cgr.dev: FORBIDDEN: Forbidden

After fix

➜  certko git:(main) ✗ ./certko --image-url=cgr.dev/chainguard/wolfi-base --ca-certs-file mycert.pem --dest-image-url docker.io/ajaykemparaj/wolfi-base:with-certs-test

Successfully appended CA certificates to image docker.io/ajaykemparaj/wolfi-base:with-certs-test
docker.io/ajaykemparaj/wolfi-base:with-certs-test@sha256:a2405d73c3d1c06b38ad80695e013720f562360aa7d2bf3e9a28960ac7288388
➜  certko git:(main) ✗ ./certko --image-url=cgr.dev/chainguard/wolfi-base --ca-certs-file mycert.pem --dest-image-url docker.io/ajaykemparaj/wolfi-base:with-certs-test-2

Successfully appended CA certificates to image docker.io/ajaykemparaj/wolfi-base:with-certs-test-2
docker.io/ajaykemparaj/wolfi-base:with-certs-test-2@sha256:a2405d73c3d1c06b38ad80695e013720f562360aa7d2bf3e9a28960ac7288388