services/galexie/docker: add multi platform support to exported dockerhub image

[sreuland]

What problem does your feature solve?

the stellar/stellar-galexie is exported in linux/amd64 arch format,
clients on arm64 architectures should include --platform linux/amd64 on the docker run stellar/stellar-galexie invocation of the image, if not included, will see a warning when running this image:

WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested

the image is able to run and finish on arm64, this is a low priority request for consideration of multi-platform support to provide slightly more seamless docker experience regardless of host system.

What would you like to see?

the stellar/stellar-galexie is exported as multi-platform manifest, including linux/arm64 as well, hosts on arm64 processors like M-series apple laptop are able to rundocker run stellar/stellar-galexie <commands> <params> w/o any warnings.

What alternatives are there?

continue with current README recommendation for manually including --platform linux/amd54 override on docker run usage.