build: create amd64/arm64 compatible docker images (#23)

docker/Dockerfile

@@ -1,5 +1,8 @@
 FROM sc4h/alpine-s6overlay:3.12
 
+ARG TARGETOS
+ARG TARGETARCH
+
 ENV \
   PATH="/app/nabarr:${PATH}" \
   APP_CONFIG="/config/config.yml" \
@@ -8,7 +11,7 @@ ENV \
   APP_VERBOSITY="0"
 
 # Binary
-COPY ["dist/nabarr_linux_amd64/nabarr", "/app/nabarr/nabarr"]
+COPY ["dist/nabarr_${TARGETOS}_${TARGETARCH}/nabarr", "/app/nabarr/nabarr"]
 
 # Add root files
 COPY ["docker/run", "/etc/services.d/nabarr/run"]