Use Docker Buildx for multi-arch builds

The bitwarden_rs code is still cross-compiled exactly as before, but Docker
Buildx is used to rewrite the resulting Docker images with correct platform
metadata (reflecting the target platform instead of the build platform).
Buildx also now handles building and pushing the multi-arch manifest lists.

docker/Dockerfile.buildx

@@ -0,0 +1,33 @@
+# The cross-built images have the build arch (`amd64`) embedded in the image
+# manifest, rather than the target arch. For example:
+#
+#   $ docker inspect bitwardenrs/server:latest-armv7 | jq -r '.[]|.Architecture'
+#   amd64
+#
+# Recent versions of Docker have started printing a warning when the image's
+# claimed arch doesn't match the host arch. For example:
+#
+#   WARNING: The requested image's platform (linux/amd64) does not match the
+#   detected host platform (linux/arm/v7) and no specific platform was requested
+#
+# The image still works fine, but the spurious warning creates confusion.
+#
+# Docker doesn't seem to provide a way to directly set the arch of an image
+# at build time. To resolve the build vs. target arch discrepancy, we use
+# Docker Buildx to build a new set of images with the correct target arch.
+#
+# Docker Buildx uses this Dockerfile to build an image for each requested
+# platform. Since the Dockerfile basically consists of a single `FROM`
+# instruction, we're effectively telling Buildx to build a platform-specific
+# image by simply copying the existing cross-built image and setting the
+# correct target arch as a side effect.
+#
+# References:
+#
+# - https://docs.docker.com/buildx/working-with-buildx/#build-multi-platform-images
+# - https://docs.docker.com/engine/reference/builder/#automatic-platform-args-in-the-global-scope
+# - https://docs.docker.com/engine/reference/builder/#understand-how-arg-and-from-interact
+#
+ARG LOCAL_REPO
+ARG DOCKER_TAG
+FROM ${LOCAL_REPO}:${DOCKER_TAG}-${TARGETARCH}${TARGETVARIANT}

docker/Dockerfile.j2

@@ -7,24 +7,24 @@
 {%     set build_stage_base_image = "clux/muslrust:nightly-2020-11-22" %}
 {%     set runtime_stage_base_image = "alpine:3.12" %}
 {%     set package_arch_target = "x86_64-unknown-linux-musl" %}
-{%   elif "arm32v7" in target_file %}
+{%   elif "armv7" in target_file %}
 {%     set build_stage_base_image = "messense/rust-musl-cross:armv7-musleabihf" %}
 {%     set runtime_stage_base_image = "balenalib/armv7hf-alpine:3.12" %}
 {%     set package_arch_target = "armv7-unknown-linux-musleabihf" %}
 {%   endif %}
 {% elif "amd64" in target_file %}
 {%   set runtime_stage_base_image = "debian:buster-slim" %}
-{% elif "arm64v8" in target_file %}
+{% elif "arm64" in target_file %}
 {%   set runtime_stage_base_image = "balenalib/aarch64-debian:buster" %}
 {%   set package_arch_name = "arm64" %}
 {%   set package_arch_target = "aarch64-unknown-linux-gnu" %}
 {%   set package_cross_compiler = "aarch64-linux-gnu" %}
-{% elif "arm32v6" in target_file %}
+{% elif "armv6" in target_file %}
 {%   set runtime_stage_base_image = "balenalib/rpi-debian:buster" %}
 {%   set package_arch_name = "armel" %}
 {%   set package_arch_target = "arm-unknown-linux-gnueabi" %}
 {%   set package_cross_compiler = "arm-linux-gnueabi" %}
-{% elif "arm32v7" in target_file %}
+{% elif "armv7" in target_file %}
 {%   set runtime_stage_base_image = "balenalib/armv7hf-debian:buster" %}
 {%   set package_arch_name = "armhf" %}
 {%   set package_arch_target = "armv7-unknown-linux-gnueabihf" %}
@@ -178,7 +178,7 @@ RUN touch src/main.rs
 # your actual source files being built
 RUN cargo build --features ${DB} --release{{ package_arch_target_param }}
 {% if "alpine" in target_file %}
-{%   if "arm32v7" in target_file %}
+{%   if "armv7" in target_file %}
 RUN musl-strip target/{{ package_arch_target }}/release/bitwarden_rs
 {%   endif %}
 {% endif %}
@@ -225,7 +225,7 @@ RUN apt-get update && apt-get install -y \
     libpq5 \
     && rm -rf /var/lib/apt/lists/*
 {% endif %}
-{% if "alpine" in target_file and "arm32v7" in target_file %}
+{% if "alpine" in target_file and "armv7" in target_file %}
 RUN apk add --no-cache -X http://dl-cdn.alpinelinux.org/alpine/edge/community catatonit
 {% endif %}
 
@@ -256,7 +256,7 @@ HEALTHCHECK --interval=60s --timeout=10s CMD ["/healthcheck.sh"]
 
 # Configures the startup!
 WORKDIR /
-{% if "alpine" in target_file and "arm32v7" in target_file %}
+{% if "alpine" in target_file and "armv7" in target_file %}
 CMD ["catatonit", "/start.sh"]
 {% else %}
 CMD ["/start.sh"]