fix: add --load flag to docker buildx build

[philasmar]

Issue #, if available:
aws/aws-toolkit-visual-studio#521

Description of changes:
Add the --load flag in docker buildx build because, unlike the standard docker build command, buildx doesn't automatically load the resulting image into the local Docker image store. When using buildx with certain docker drivers (like docker-container), the build process happens in a container, and the resulting image needs to be explicitly loaded into the local image store using --load.

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[Copilot]

Pull Request Overview

This PR adds the --load flag to the docker buildx build command so that built images are automatically loaded into the local Docker image store when using buildx.

• Update the container build command to include --load for cross-architecture builds
• Add a changelog entry describing the new flag

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/AWS.Deploy.Orchestration/DeploymentBundleHandler.cs	Include --load flag in buildx build command
.autover/changes/aeb80c58-0a88-4bcb-9fc4-c23c4045e6ec.json	Add changelog entry for the new --load flag

Comments suppressed due to low confidence (1)

src/AWS.Deploy.Orchestration/DeploymentBundleHandler.cs:69

• Add a unit test to verify that when building cross-architecture images, the generated buildCommand string includes the --load flag to prevent regressions.

                buildCommand = $"{commandName} buildx build --load --platform {platform} -t {imageTag} -f \"{dockerFile}\"{buildArgs} .";

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.10%. Comparing base (7aa80c4) to head (99f7de5).
Report is 11 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #966      +/-   ##
==========================================
- Coverage   62.14%   62.10%   -0.04%     
==========================================
  Files         294      294              
  Lines       10796    10796              
  Branches     1616     1616              
==========================================
- Hits         6709     6705       -4     
- Misses       3487     3490       +3     
- Partials      600      601       +1     

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