Add support for different architecture

[jjyong90]

Downloaded images are for the ARM64 architecture by default.
But docker pull accepts a --platform argument which can be used.

Expected behavior:

curl https://dockerimagesave.akiel.dev/pull/alpine:latest?platform=linux/amd64
#or
curl https://dockerimagesave.akiel.dev/pull/alpine:latest?platform=linux%2Famd64

Then docker alpine image is downloaded for the linux/amd64 architecture.

[stescobedo92]

Fixed with PR: #191