fix(docker): fix base image for multi-platform build (runatlantis#2099)

* Correct indentation of run commands

* Split installation of packages into the ones needed at run time and build time

This allows us to now repeat the packages which need to be uninstalled again by
making use of a virtual package, which - when removed - removes the packages
installed as a dependency of it.

* Remove unnecessary `rm -rf /var/cache/apk/*` command

It's no needed when `apt add` is run with the `--no-cache` option.

* Add vertical spacing so it's clearer what is happening when

* Test the downloaded binaries to make sure they work on the platform

This can help find issues where binaries are downloaded for the wrong platform
compared to the architecture the Docker image is built for.

* Install dumb-init via apk

It's available as a package for Alpine Linux in version 1.2.5 as well, which
makes it easier to handle for the different architectures.

* Get git-lfs binaries in the right architecture for the Docker image

This makes use of the `TARGETPLATFORM` argument which automatically is
populated by Docker BuildKit with a string such as "linux/amd64" when the image
is being build for an x86_64 architecture.

* Install gosu for the right architecture

The `case` statement was taken from
https://github.com/BretFisher/multi-platform-docker-build as a way of
translating the platform name into what we needed for downloading gosu.

docker-base/Dockerfile

@@ -17,21 +17,35 @@ RUN addgroup atlantis && \
     chmod g=u /home/atlantis/ && \
     chmod g=u /etc/passwd
 
-# Install dumb-init, gosu and git-lfs.
-ENV DUMB_INIT_VERSION=1.2.5
+# Install gosu and git-lfs.
 ENV GOSU_VERSION=1.14
 ENV GIT_LFS_VERSION=3.1.2
-RUN apk add --no-cache ca-certificates gnupg curl git unzip bash openssh libcap openssl && \
-    curl -L -s --output /bin/dumb-init "https://github.com/Yelp/dumb-init/releases/download/v${DUMB_INIT_VERSION}/dumb-init_${DUMB_INIT_VERSION}_x86_64" && \
-    chmod +x /bin/dumb-init && \
+
+# Automatically populated with the architecture the image is being built for.
+ARG TARGETPLATFORM
+
+# Install packages needed for running Atlantis.
+RUN apk add --no-cache ca-certificates curl git unzip bash openssh libcap dumb-init && \
+    # Install packages needed for building dependencies.
+    apk add --no-cache --virtual .build-deps gnupg openssl && \
     mkdir -p /tmp/build && \
     cd /tmp/build && \
-    curl -L -s --output git-lfs.tar.gz "https://github.com/git-lfs/git-lfs/releases/download/v${GIT_LFS_VERSION}/git-lfs-linux-amd64-v${GIT_LFS_VERSION}.tar.gz" && \
+
+    # git-lfs
+    curl -L -s --output git-lfs.tar.gz "https://github.com/git-lfs/git-lfs/releases/download/v${GIT_LFS_VERSION}/git-lfs-linux-${TARGETPLATFORM##*/}-v${GIT_LFS_VERSION}.tar.gz" && \
     tar -xf git-lfs.tar.gz && \
     chmod +x git-lfs && \
     mv git-lfs /usr/bin/git-lfs && \
-    curl -L -s --output gosu "https://github.com/tianon/gosu/releases/download/${GOSU_VERSION}/gosu-amd64" && \
-    curl -L -s --output gosu.asc "https://github.com/tianon/gosu/releases/download/${GOSU_VERSION}/gosu-amd64.asc" && \
+    git-lfs --version && \
+
+    # gosu
+    case ${TARGETPLATFORM} in \
+        "linux/amd64") GOSU_ARCH=amd64 ;; \
+        "linux/arm64") GOSU_ARCH=arm64 ;; \
+        "linux/arm/v7") GOSU_ARCH=armhf ;; \
+    esac && \
+    curl -L -s --output gosu "https://github.com/tianon/gosu/releases/download/${GOSU_VERSION}/gosu-${GOSU_ARCH}" && \
+    curl -L -s --output gosu.asc "https://github.com/tianon/gosu/releases/download/${GOSU_VERSION}/gosu-${GOSU_ARCH}.asc" && \
     for server in $(shuf -e ipv4.pool.sks-keyservers.net \
                             hkp://p80.pool.sks-keyservers.net:80 \
                             keyserver.ubuntu.com \
@@ -42,13 +56,15 @@ RUN apk add --no-cache ca-certificates gnupg curl git unzip bash openssh libcap
     gpg --batch --verify gosu.asc gosu && \
     chmod +x gosu && \
     cp gosu /bin && \
-        cd /tmp && \
-        rm -rf /tmp/build && \
-        gpgconf --kill dirmngr && \
-        gpgconf --kill gpg-agent && \
-        apk del gnupg openssl && \
-        rm -rf /root/.gnupg && \
-        rm -rf /var/cache/apk/*
+    gosu --version && \
+
+    # Cleanup
+    cd /tmp && \
+    rm -rf /tmp/build && \
+    gpgconf --kill dirmngr && \
+    gpgconf --kill gpg-agent && \
+    apk del .build-deps && \
+    rm -rf /root/.gnupg
 
 # Set up nsswitch.conf for Go's "netgo" implementation
 # - https://github.com/golang/go/blob/go1.9.1/src/net/conf.go#L194-L275