Makefile: correctly annotate container image designated platform

[paulfantom]

This should fix the incorrectly assigned CPU architecture in container image manifests. This is mostly a cosmetic change as we set proper CPU architecture in a main virtual manifest.

Example manifest for ppc64le before this change:

$ manifest-tool inspect quay.io/brancz/kube-rbac-proxy:v0.9.0-ppc64le
quay.io/brancz/kube-rbac-proxy:v0.9.0-ppc64le: manifest type: application/vnd.docker.distribution.manifest.v2+json
      Digest: sha256:56673d4eed7952068952ba84219ffc6693496a2e4fef57ea9bd776fb430fb6a3
Architecture: amd64
          OS: linux
    # Layers: 2
      layer 1: digest = sha256:5dea5ec2316d4a067b946b15c3c4f140b4f2ad607e73e9bc41b673ee5ebb99a3
      layer 2: digest = sha256:ea0eb814dc9409f25a39a903ad26b0e1d137f7fbfc3e61be8adf8dddb2d2ede2

After this change:

$ manifest-tool inspect quay.io/paulfantom/kube-rbac-proxy:better-arch-2021-04-28-561752e9-ppc64le
quay.io/paulfantom/kube-rbac-proxy:better-arch-2021-04-28-561752e9-ppc64le: manifest type: application/vnd.docker.distribution.manifest.v2+json
      Digest: sha256:e24c5afd19b69d4770e2780913128c217623663a718d06189db5c8313c007721
Architecture: ppc64le
          OS: linux
    # Layers: 2
      layer 1: digest = sha256:6e9c11b56e44f7f50a9868e2f481dc7f539ad6d77a71de7e9758683bf26282dc
      layer 2: digest = sha256:e6eba0c2a36ce4e43da3df5a80c41f9034eace684811f1498e9a08c43e27dcd8

As you can see Architecture field is filed correctly.

/cc @s-urbaniak

[paulfantom]

This PR is a side effect of watching discussion from kubernetes-sigs/kubebuilder#1785 :)

[paulfantom]

To make this complete, here is our main virtual manifest which already has proper architecture settings (this one is from latest release):

$ manifest-tool inspect quay.io/brancz/kube-rbac-proxy:v0.9.0
Name:   quay.io/brancz/kube-rbac-proxy:v0.9.0 (Type: application/vnd.docker.distribution.manifest.list.v2+json)
Digest: sha256:836fb4215f60a77f592a46d459067814ef306fb5409d75730c63b7572ebe1509
 * Contains 5 manifest references:
1    Mfst Type: application/vnd.docker.distribution.manifest.v2+json
1       Digest: sha256:cc0aba33fe5083c3d23c463a84b64d1cd921434b0e530418c7e8f08855c1e28f
1  Mfst Length: 739
1     Platform:
1           -      OS: linux
1           - OS Vers: 
1           - OS Feat: []
1           -    Arch: amd64
1           - Variant: 
1           - Feature: 
1     # Layers: 2
         layer 1: digest = sha256:5dea5ec2316d4a067b946b15c3c4f140b4f2ad607e73e9bc41b673ee5ebb99a3
         layer 2: digest = sha256:0cb54490015b1c8bc1318e4eeac58f0ac7452bbf992ba7a0f982eacf407b3db6

2    Mfst Type: application/vnd.docker.distribution.manifest.v2+json
2       Digest: sha256:b2e86f12ec25b2789fcbbd0295cf4ba578ce45d5f2ff953154dfa2e770c2a291
2  Mfst Length: 739
2     Platform:
2           -      OS: linux
2           - OS Vers: 
2           - OS Feat: []
2           -    Arch: arm
2           - Variant: 
2           - Feature: 
2     # Layers: 2
         layer 1: digest = sha256:5dea5ec2316d4a067b946b15c3c4f140b4f2ad607e73e9bc41b673ee5ebb99a3
         layer 2: digest = sha256:70b2107db14c555cf57017c4b259adedba94e732bc0e7070650a9495eaa61065

3    Mfst Type: application/vnd.docker.distribution.manifest.v2+json
3       Digest: sha256:b5018990dab8c3d39bdd51db046d7cae0b757dc12ff7cee567c61cbb64571a64
3  Mfst Length: 739
3     Platform:
3           -      OS: linux
3           - OS Vers: 
3           - OS Feat: []
3           -    Arch: arm64
3           - Variant: 
3           - Feature: 
3     # Layers: 2
         layer 1: digest = sha256:5dea5ec2316d4a067b946b15c3c4f140b4f2ad607e73e9bc41b673ee5ebb99a3
         layer 2: digest = sha256:99cfe4c126e3c7085373b5b8ae084ef05a41eba38a717e9f54befd41aeebfd4d

4    Mfst Type: application/vnd.docker.distribution.manifest.v2+json
4       Digest: sha256:56673d4eed7952068952ba84219ffc6693496a2e4fef57ea9bd776fb430fb6a3
4  Mfst Length: 739
4     Platform:
4           -      OS: linux
4           - OS Vers: 
4           - OS Feat: []
4           -    Arch: ppc64le
4           - Variant: 
4           - Feature: 
4     # Layers: 2
         layer 1: digest = sha256:5dea5ec2316d4a067b946b15c3c4f140b4f2ad607e73e9bc41b673ee5ebb99a3
         layer 2: digest = sha256:ea0eb814dc9409f25a39a903ad26b0e1d137f7fbfc3e61be8adf8dddb2d2ede2

5    Mfst Type: application/vnd.docker.distribution.manifest.v2+json
5       Digest: sha256:d4fc3dae5266d6b9a1662d79cac9f2c4fd78d7f4ef6494c6c650107e22e0b7a2
5  Mfst Length: 739
5     Platform:
5           -      OS: linux
5           - OS Vers: 
5           - OS Feat: []
5           -    Arch: s390x
5           - Variant: 
5           - Feature: 
5     # Layers: 2
         layer 1: digest = sha256:5dea5ec2316d4a067b946b15c3c4f140b4f2ad607e73e9bc41b673ee5ebb99a3
         layer 2: digest = sha256:e3e860aecefbe1d9d9adea667c19d69f2e35cb522707a92af349e7cfa1e08390

[s-urbaniak]

@paulfantom nice, thank you! lgtm 👍