Add configuration to allow for `aarch64` hosts in Dockerfile

[Zacharias]

Spruce is pulled in to the Dockerfile as a binary dependency. It is used in entrypoint.sh to merge configuration files to control the behavior of scytale.

It's sourcing can be seen here:

# https://github.com/xmidt-org/scytale/blob/main/Dockerfile#L21
# Download spruce here to eliminate the need for curl in the final image
RUN mkdir -p /go/bin && \
    curl -L -o /go/bin/spruce https://github.com/geofffranks/spruce/releases/download/v1.29.0/spruce-linux-amd64 && \
    chmod +x /go/bin/spruce

Since we want the ability to build and run scrytale on a variety of processor architectures, we need to find a way to get a processor-architecture-specific copy of spruce installed. This could added by adding configuration around processor architectures, or alternatively building spruce from source and including it in the dockerfile.

[Zacharias]

Did some looking into https://github.com/geofffranks/spruce/releases/ and I do see a arm64 download listed, which is a now-deprecated nomenclature for the aarch64 ISA. The docker image for golang shows the following when ran:

% docker run -it golang:1.19-alpine /bin/sh
/go # uname -a
Linux ff721415d026 6.2.13-300.fc38.aarch64 #1 SMP PREEMPT_DYNAMIC Thu Apr 27 01:54:55 UTC 2023 aarch64 Linux
/go # uname -m
aarch64
/go # exit