Add multi-architecture Docker image build

* Adds a shell script with functions to install multi-architecture docker
  support, as well as build, deploy, and test the shellcheck docker images for
  the same set of architectures for which binaries were already built and
  deployed as tarballs.
* Hooks up the multi-architecture docker build, deploy, and test to the existing
  Travis CI/CD pipeline. It is organized as a separate stage which only runs if
  all previous steps in the already existing test stage succeed.

.travis.yml

@@ -6,13 +6,19 @@ services:
 
 jobs:
   include:
-    - env: BUILD=linux
+    - stage: Test
+      env: BUILD=linux
     - env: BUILD=windows
     - env: BUILD=armv6hf
     - env: BUILD=aarch64
     - env: BUILD=osx
       os: osx
 
+    - stage: Deploy docker image
+      script:
+        - source ./.multi_arch_docker
+        - set -ex; multi_arch_docker::main; set +x
+
 before_install: |
   DOCKER_BASE="$DOCKER_USERNAME/shellcheck"
   DOCKER_BUILDS=""
@@ -28,18 +34,6 @@ script:
   - set -ex; build_"$BUILD"; set +x;
   - ./.prepare_deploy
 
-after_success: |
-  if [ "$BUILD" = "linux" ]; then
-    docker login -u="$DOCKER_USERNAME" -p="$DOCKER_PASSWORD"
-    for repo in $DOCKER_BUILDS; do
-      for tag in $TAGS; do
-        echo "Deploying $repo:current as $repo:$tag...";
-        docker tag "$repo:current" "$repo:$tag" || exit 1;
-        docker push "$repo:$tag" || exit 1;
-      done;
-    done;
-  fi
-
 after_failure: |
   id
   pwd
@@ -57,4 +51,5 @@ deploy:
   local_dir: deploy
   on:
     repo: koalaman/shellcheck
+    condition: $TRAVIS_BUILD_STAGE_NAME = Test
     all_branches: true