dockerfile: Fix multi-platform builds (#105)

* Fix bugs introduced by #103
* Add missing dependency for `libatomic1` which is need at least by `riscv64`

Build command:

    docker buildx build --platform linux/arm64,linux/amd64,linux/riscv64 --pull .
    image=test
    docker buildx build --platform linux/amd64 -t $image --load .
    docker run --rm -it $image --version

Author: kylemanna

Dockerfile

@@ -2,26 +2,27 @@
 # Alpine would be nice, but it's linked again musl and breaks the bitcoin core download binary
 #FROM alpine:latest
 
-FROM ubuntu:latest as builder
+FROM ubuntu:latest AS builder
 ARG TARGETARCH
 
-FROM builder as builder_x86_64
+FROM builder AS builder_amd64
 ENV ARCH=x86_64
-FROM builder as builder_arm
-ENV ARCH=arm
-FROM builder as builder_rscv64
+FROM builder AS builder_arm64
+ENV ARCH=aarch64
+FROM builder AS builder_riscv64
 ENV ARCH=riscv64
 
-FROM builder_${TARGETARCH}
+FROM builder_${TARGETARCH} AS build
 
 # Testing: gosu
 #RUN echo "http://dl-cdn.alpinelinux.org/alpine/edge/testing/" >> /etc/apk/repositories \
 #    && apk add --update --no-cache gnupg gosu gcompat libgcc
 RUN apt update \
     && apt install -y --no-install-recommends \
     ca-certificates \
-    wget \
     gnupg \
+    libatomic1 \
+    wget \
     && apt clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
 ARG VERSION=24.0
@@ -58,10 +59,10 @@ ARG USER_ID=1000
 RUN groupadd -g ${GROUP_ID} bitcoin \
     && useradd -u ${USER_ID} -g bitcoin -d /bitcoin bitcoin
 
-COPY --from=builder /opt/ /opt/
+COPY --from=build /opt/ /opt/
 
 RUN apt update \
-    && apt install -y --no-install-recommends gosu \
+    && apt install -y --no-install-recommends gosu libatomic1 \
     && apt clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* \
     && ln -sv /opt/bitcoin/bin/* /usr/local/bin