proxy: enable ARCH switch in Dockerfile

.drone.yml

@@ -222,6 +222,8 @@ steps:
         from_secret: docker_username
       password:
         from_secret: docker_password
+      build_args:
+        - ARCH=arm64
     when:
       event:
         - tag
@@ -257,6 +259,8 @@ steps:
         from_secret: docker_username
       password:
         from_secret: docker_password
+      build_args:
+        - ARCH=arm64
     when:
       event:
         - tag

proxy/Dockerfile

@@ -1,8 +1,12 @@
 FROM nginx:1.16.0-alpine
-
+ARG ARCH
+# set ARCH from build-arg, if it's set
+ENV ARCH=${ARCH}
+# if ARCH unset, default to amd64
+ENV ARCH=${ARCH:-amd64}
 RUN apk -U --no-cache add curl ca-certificates\
     && mkdir -p /etc/confd \
-    && curl -sLf https://github.com/kelseyhightower/confd/releases/download/v0.16.0/confd-0.16.0-linux-amd64 > /usr/bin/confd \
+    && curl -sLf https://github.com/kelseyhightower/confd/releases/download/v0.16.0/confd-0.16.0-linux-${ARCH} > /usr/bin/confd \
     && chmod +x /usr/bin/confd \
     && apk del curl