Simplify the Dockerfile (#13974)

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# Description

Works for `docker build` and `docker buildx build`
For more detail about the new Dockerfile syntax:
https://www.docker.com/blog/new-dockerfile-capabilities-v1-7-0/

docker/Dockerfile

@@ -1,3 +1,5 @@
+# syntax=docker/dockerfile:latest
+
 # Git: git version 2.30.2
 # /etc/os-release: Alpine Linux v3.16
 # Kernel: Linux ca3abedc4fb1 5.17.15-76051715-generic #202206141358~1655919116~22.04~1db9e34 SMP PREEMPT Wed Jun 22 19 x86_64 Linux
@@ -8,25 +10,17 @@ FROM alpine
 LABEL maintainer=nushell
 
 ARG TARGETARCH
+ARG ARCH=${TARGETARCH/arm64/aarch64}
+ARG ARCH=${ARCH/amd64/x86_64}
 
-RUN set -eux; \
-    if [ "${TARGETARCH}" = "amd64" ] || [ "${TARGETARCH}" = "x86_64" ]; then \
-        echo "Downloading x86_64 binary for ${TARGETARCH}..."; \
-        arch_path="x86_64"; \
-    elif [ "${TARGETARCH}" = "arm64" ] || [ "${TARGETARCH}" = "aarch64"]; then \
-        echo "Downloading aarch64 binary for ${TARGETARCH}..."; \
-        arch_path="aarch64"; \
-    else \
-        arch_path=""; \
-    fi; \
-    echo '/usr/bin/nu' >> /etc/shells \
+RUN echo '/usr/bin/nu' >> /etc/shells \
     && adduser -D -s /usr/bin/nu nushell \
     && mkdir -p /home/nushell/.config/nushell/ \
     && cd /tmp \
     && wget -qO - https://api.github.com/repos/nushell/nushell/releases/latest \
     | grep browser_download_url \
-    | grep "${arch_path}.*.musl.tar.gz" \
-    | cut -f4 -d '"' \
+    | cut -d '"' -f 4 \
+    | grep ${ARCH}-unknown-linux-musl \
     | xargs -I{} wget {} \
     && tar -xzf nu* \
     && cd nu*-musl \