Fix i386 "softfloat" bug

[tianon]

See golang/go#44500 for details (tldr, Go 1.16 changes "GO386" from "387" to "softfloat" but the bootstrap compiler isn't 1.16 yet so balks at the changed value).

This also makes our "compilation" much more explicit by explicitly noting the "env" needed to build each bashbrew architecture we support and embedding those directly.

(I've also added "i386" to GitHub Actions, which will double our matrix size, but will hopefully help us catch issues like this in the future.)

Fixes #359

[tianon]

Diff:

$ diff -u --ignore-all-space Dockerfile-debian.template Dockerfile-alpine.template
--- Dockerfile-debian.template	2021-02-24 08:44:02.632372033 -0800
+++ Dockerfile-alpine.template	2021-02-24 08:34:24.736383847 -0800
@@ -1,71 +1,64 @@
-FROM buildpack-deps:{{ env.variant }}-scm
+FROM alpine:{{ env.variant | ltrimstr("alpine") }}
 
-# gcc for cgo
-RUN apt-get update && apt-get install -y --no-install-recommends \
-		g++ \
-		gcc \
-		libc6-dev \
-		make \
-		pkg-config \
-	&& rm -rf /var/lib/apt/lists/*
+RUN apk add --no-cache \
+		ca-certificates
+
+# set up nsswitch.conf for Go's "netgo" implementation
+# - https://github.com/golang/go/blob/go1.9.1/src/net/conf.go#L194-L275
+# - docker run --rm debian:stretch grep '^hosts:' /etc/nsswitch.conf
+RUN [ ! -e /etc/nsswitch.conf ] && echo 'hosts: files dns' > /etc/nsswitch.conf
 
 ENV PATH /usr/local/go/bin:$PATH
 
 ENV GOLANG_VERSION {{ .version }}
 
 RUN set -eux; \
-	\
-	dpkgArch="$(dpkg --print-architecture)"; \
-	url=; \
-	case "${dpkgArch##*-}" in \
+	apk add --no-cache --virtual .build-deps \
+		bash \
+		gcc \
+		gnupg \
+		go \
+		musl-dev \
+		openssl \
+	; \
+	apkArch="$(apk --print-arch)"; \
+	case "$apkArch" in \
 {{
 	[
 		.arches | to_entries[]
 		| .key as $bashbrewArch
 		| (
 			{
-				amd64: "amd64",
-				arm32v5: "armel",
-				arm32v7: "armhf",
-				arm64v8: "arm64",
-				i386: "i386",
-				mips64le: "mips64el",
-				ppc64le: "ppc64el",
+				amd64: "x86_64",
+				arm32v6: "armhf",
+				arm32v7: "armv7",
+				arm64v8: "aarch64",
+				i386: "x86",
+				ppc64le: "ppc64le",
 				s390x: "s390x",
 			} | .[$bashbrewArch] // ""
-		) as $dpkgArch
-		| select($dpkgArch != "")
+		) as $apkArch
+		| select($apkArch != "")
 		| .value
 		| (
 -}}
-		{{ $dpkgArch | @sh }}) \
-{{ if .url then ( -}}
-			url={{ .url | @sh }}; \
-			sha256={{ .sha256 | @sh }}; \
-{{ ) else ( -}}
+		{{ $apkArch | @sh }}) \
 			export {{ .env | to_entries | map(.key + "=" + (.value | @sh)) | join(" ") }}; \
-{{ ) end -}}
 			;; \
 {{
 		)
 	] | add
 -}}
-		*) echo >&2 "error: unsupported architecture '$dpkgArch' (likely packaging update needed)"; exit 1 ;; \
+		*) echo >&2 "error: unsupported architecture '$apkArch' (likely packaging update needed)"; exit 1 ;; \
 	esac; \
-	build=; \
-	if [ -z "$url" ]; then \
+	\
 # https://github.com/golang/go/issues/38536#issuecomment-616897960
-		build=1; \
 		url={{ .arches.src.url | @sh }}; \
 		sha256={{ .arches.src.sha256 | @sh }}; \
-		echo >&2; \
-		echo >&2 "warning: current architecture ($dpkgArch) does not have a corresponding Go binary release; will be building from source"; \
-		echo >&2; \
-	fi; \
 	\
-	wget -O go.tgz.asc "$url.asc" --progress=dot:giga; \
-	wget -O go.tgz "$url" --progress=dot:giga; \
-	echo "$sha256 *go.tgz" | sha256sum --strict --check -; \
+	wget -O go.tgz.asc "$url.asc"; \
+	wget -O go.tgz "$url"; \
+	echo "$sha256 *go.tgz" | sha256sum -c -; \
 	\
 # https://github.com/golang/go/issues/14739#issuecomment-324767697
 	export GNUPGHOME="$(mktemp -d)"; \
@@ -78,22 +71,24 @@
 	tar -C /usr/local -xzf go.tgz; \
 	rm go.tgz; \
 	\
-	if [ -n "$build" ]; then \
-		savedAptMark="$(apt-mark showmanual)"; \
-		apt-get update; \
-		apt-get install -y --no-install-recommends golang-go; \
-		\
 		( \
 			cd /usr/local/go/src; \
 # set GOROOT_BOOTSTRAP + GOHOST* such that we can build Go successfully
 			export GOROOT_BOOTSTRAP="$(go env GOROOT)" GOHOSTOS="$GOOS" GOHOSTARCH="$GOARCH"; \
+		if [ "${GO386:-}" = 'softfloat' ]; then \
+# https://github.com/docker-library/golang/issues/359 -> https://github.com/golang/go/issues/44500
+# (once our Alpine base has Go 1.16, we can remove this hack)
+			GO386= ./bootstrap.bash; \
+			export GOROOT_BOOTSTRAP="/usr/local/go-$GOOS-$GOARCH-bootstrap"; \
+			"$GOROOT_BOOTSTRAP/bin/go" version; \
+		fi; \
 			./make.bash; \
+		if [ "${GO386:-}" = 'softfloat' ]; then \
+			rm -rf "$GOROOT_BOOTSTRAP"; \
+		fi; \
 		); \
 		\
-		apt-mark auto '.*' > /dev/null; \
-		apt-mark manual $savedAptMark > /dev/null; \
-		apt-get purge -y --auto-remove -o APT::AutoRemove::RecommendsImportant=false; \
-		rm -rf /var/lib/apt/lists/*; \
+	apk del --no-network .build-deps; \
 		\
 # pre-compile the standard library, just like the official binary release tarballs do
 		go install std; \
@@ -109,7 +104,6 @@
 			/usr/local/go/pkg/tool/*/go_bootstrap \
 			/usr/local/go/src/cmd/dist/dist \
 		; \
-	fi; \
 	\
 	go version