fix: add --load flag to docker buildx build to enable image pushing

[dhvanipa]

Motivation:
If the user manually deploys an image with eigenx app deploy and it doesn't have EigenX components, the CLI adds them, if the user replies Yes to Would you like to layer the image with EigenX components?. However, the CLI fails to push the new image it creates.

Modifications:
Added the missing --load flag to docker buildx. This flag tells Docker buildx to load the built image into the local Docker daemon, making it available for the subsequent docker push operation. Without this flag, buildx only stores the image in the build cache and doesn't make it available for pushing.

Result:
Users can successfully deploy a pre-built image that doesn't have the EigenX components.

Testing:
Previously, eigenx app deploy dhvanipa/mc-server:1.21.10 would fail with 2025/11/11 09:47:38 failed to ensure image compatibility: failed to layer published image: failed to push layered image: build error: tag does not exist: dhvanipa/mc-server:1.21.10-eigenx

Open questions:
N/A

[solimander]

Thanks for the contribution! Merging in #38 as we require signed commits.