Provide an actual multi-arch Docker build

Removed all hard-coded references to aarch64. The local aarch64 build
still works, let's see what happens with the GitHub workflow...

.dockerignore

@@ -0,0 +1,2 @@
+.git/
+node_modules/

Dockerfile

@@ -1,4 +1,4 @@
-FROM arm64v8/node:12-alpine3.12 AS npm-ci
+FROM node:12-alpine3.12 AS npm-ci
 
 SHELL ["/bin/ash", "-euo", "pipefail", "-c"]
 
@@ -9,18 +9,25 @@ COPY package*.json *.js LICENSE ./
 RUN apk add --no-cache python3~=3.8 make~=4.3 g++~=9.3 && \
     npm ci --production
 
-FROM arm64v8/node:12-alpine3.12
+FROM node:12-alpine3.12
 
 SHELL ["/bin/ash", "-euo", "pipefail", "-c"]
 
 RUN apk add --no-cache --virtual /tmp/.gpg gnupg~=2.2 && \
+    # Set correct architecture for s6-overlay
+    case $(arch) in \
+      aarch64) arch=aarch64 ;; \
+      armv7l)  arch=armhf ;; \
+      x86_64)  arch=amd64 ;; \
+      *) echo "Invalid architecture '$(arch)'" && exit 1 ;; \
+    esac && \
     # Download just-containers s6-overlay installer and its signature
-    wget -O /tmp/s6-installer \
-      https://github.com/just-containers/s6-overlay/releases/download/v2.1.0.2/s6-overlay-aarch64-installer && \
-    wget -O /tmp/s6-installer.sig \
-      https://github.com/just-containers/s6-overlay/releases/download/v2.1.0.2/s6-overlay-aarch64-installer.sig && \
+    wget -nv -O /tmp/s6-installer \
+      "https://github.com/just-containers/s6-overlay/releases/download/v2.1.0.2/s6-overlay-$arch-installer" && \
+    wget -nv -O /tmp/s6-installer.sig \
+      "https://github.com/just-containers/s6-overlay/releases/download/v2.1.0.2/s6-overlay-$arch-installer.sig" && \
     # Import just-containers' public key; use gpgv to validate installer
-    wget -O - https://keybase.io/justcontainers/pgp_keys.asc?fingerprint=db301ba3f6f807e0d0e6ccb86101b2783b2fd161 | \
+    wget -nv -O - https://keybase.io/justcontainers/pgp_keys.asc?fingerprint=db301ba3f6f807e0d0e6ccb86101b2783b2fd161 | \
       gpg --no-default-keyring --keyring /tmp/s6-installer.gpg --import && \
     gpgv --keyring /tmp/s6-installer.gpg /tmp/s6-installer.sig /tmp/s6-installer && \
     # Execute the installer

TODO

@@ -12,7 +12,3 @@
       (regardless of the script's exit code)
   - Build some retry logic into the s6-overlay scripts? Or find a way to
     "escalate" the problem out of the container?
-- [ ] The multi-arch Dockers don't actually work... 😳
-  - The `📄 Dockerfile` is hardcoded to using `arm64v8/node:12-alpine3.12` as
-    its base image _and_ I'm pulling in the `s6-overlay-aarch64-installer`
-    regardless of the target architecture... 🤐