Mutli-arch support

[tejal29]

Add multi-arch support for all kaniko images.

Prior to this PR, amazing @mattmoor added multi-arch to the executor image via bazel so that tekton users can use kaniko multi-arch images.
The arch specific images did not contain credential binary for gcr, ecr and aws. After 1:1 conversation with Matt, the reason why they were excluded is, tekton already sets up auth prior to kaniko pod is created.

In this PR,

• use docker buildx in cloudbuild.yaml to create multi-arch images for executor:debug as well as warmer.
• publish new, executor:slim & executor:$TAG-slim` images which only contains kaniko binary and config files.
• remove bazel targets and use docker buildx to create image for executor like others

To do

• run the cloud build trigger & verify the images.
• ping @mattmoor to test/verify executor:slim images - optional
• ping kaniko slack channel to test debug image - optional

[tejal29]

/cc @briandealwis for an extra pair of eyes.

[mattmoor]

cc @imjasonh

I hadn't seen buildx on cloud build before. neat!

[priyawadhwa]

LGTM, just one question

[briandealwis]

LGTM

[tejal29]

Testing Notes: Executor Image

➜  kaniko git:(fix_cloudbuild_release) ✗ container-diff diff gcr.io/kaniko-project/executor gcr.io/tejal-test/executor:multiarch-test-1 --type=file

-----File-----

These entries have been added to gcr.io/kaniko-project/executor:
FILE         SIZE
/dev         0
/proc        0
/sys         0

These entries have been deleted from gcr.io/kaniko-project/executor:
FILE                                     SIZE
/kaniko/docker-credential-acr-env        8.7M

These entries have been changed between gcr.io/kaniko-project/executor and gcr.io/tejal-test/executor:multiarch-test-1:
FILE                                       SIZE1        SIZE2
/kaniko/executor                           47.2M        47.6M
/kaniko/docker-credential-acr              8.1M         9.9M
/kaniko/docker-credential-ecr-login        8M           9M
/kaniko/docker-credential-gcr              6.2M         8.9M

➜  kaniko git:(fix_cloudbuild_release) ✗ dive gcr.io/tejal-test/executor:multiarch-test-1
Image Source: docker://gcr.io/tejal-test/executor:multiarch-test-1
Fetching image... (this can take a while for large images)
Analyzing image...
Building cache...

The manifest generated is of type manifest.list.v2+json

[tejal29]

Testing Notes: executor:slim image

[tejal29]

Testing Notes: Debug image

➜  kaniko git:(fix_cloudbuild_release) ✗ container-diff diff gcr.io/kaniko-project/executor:debug gcr.io/tejal-test/executor:multiarch-test-1-debug --type=file

-----File-----

These entries have been added to gcr.io/kaniko-project/executor:debug:
FILE         SIZE
/dev         0
/proc        0
/sys         0

These entries have been deleted from gcr.io/kaniko-project/executor:debug: None

These entries have been changed between gcr.io/kaniko-project/executor:debug and gcr.io/tejal-test/executor:multiarch-test-1-debug:
FILE                                       SIZE1        SIZE2
/kaniko/executor                           47.2M        47.6M
/kaniko/warmer                             31.5M        31.7M
/kaniko/docker-credential-acr-env          8.7M         13.2M
/kaniko/docker-credential-acr              8.1M         9.9M
/kaniko/docker-credential-ecr-login        8M           9M
/kaniko/docker-credential-gcr              6.2M         8.9M

➜  kaniko git:(fix_cloudbuild_release) ✗

[tejal29]

Testing Note: Warmer image

container-diff diff gcr.io/kaniko-project/warmer gcr.io/tejal-test/warmer:multiarch-test-1 --type=file 

-----File-----

These entries have been added to gcr.io/kaniko-project/warmer:
FILE                                     SIZE
/dev                                     0
/kaniko/docker-credential-acr-env        13.2M
/proc                                    0
/sys                                     0

These entries have been deleted from gcr.io/kaniko-project/warmer: None

These entries have been changed between gcr.io/kaniko-project/warmer and gcr.io/tejal-test/warmer:multiarch-test-1:
FILE                                       SIZE1        SIZE2
/kaniko/warmer                             31.5M        31.7M
/kaniko/docker-credential-acr              8.1M         9.9M
/kaniko/docker-credential-ecr-login        8M           9M
/kaniko/docker-credential-gcr              6.1M         8.9M

[tejal29]

Images for test

gcr.io/tejal-test/executor:multiarch-test-1
gcr.io/tejal-test/warmer:multiarch-test-1-debug
gcr.io/tejal-test/executor:multiarch-test-1-debug
gcr.io/tejal-test/executor:multiarch-test-1-slim

[tejal29]

Fixed slim images not containing the SSL certs /cc @mattmoor

gcr.io/tejal-test/executor:multiarch-test-2-slim

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