🌱 change casing from as to AS in Dockerfile

[kranurag7]

this was giving me warning locally and this commit fixes the same. Docker version used locally is 27.0.3

$ docker buildx build -f images/caph/Dockerfile . --check
[+] Building 0.9s (5/5) FINISHED                                                                                                                 docker:default
 => [internal] connecting to local controller                                                                                                              0.0s
 => [internal] load build definition from Dockerfile                                                                                                       0.0s
 => => transferring dockerfile: 1.57kB                                                                                                                     0.0s
 => [internal] load metadata for gcr.io/distroless/static:nonroot                                                                                          0.8s
 => [internal] load metadata for docker.io/library/golang:1.21.6-bullseye@sha256:a8712f27d9ac742e7bded8f81f7547c5635e855e8b80302e8fc0ce424f559295          0.0s
 => [internal] load .dockerignore                                                                                                                          0.0s
 => => transferring context: 252B                                                                                                                          0.0s

WARNING: FromAsCasing - https://docs.docker.com/go/dockerfile/rule/from-as-casing/
'as' and 'FROM' keywords' casing do not match
Dockerfile:16
--------------------
  14 |
  15 |     # Build the manager binary
  16 | >>> FROM --platform=${BUILDPLATFORM} docker.io/library/golang:1.21.6-bullseye@sha256:a8712f27d9ac742e7bded8f81f7547c5635e855e8b80302e8fc0ce424f559295 as build
  17 |     ARG TARGETOS TARGETARCH
  18 |
--------------------

Ref: https://docs.docker.com/reference/build-checks/from-as-casing/ (the casing should be the same)

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squash commits
• include documentation
• add unit tests

[batistein]

interesting finding! Maybe it makes sense to have this integrated in our linting.