ci: Don't use Travis caches for docker images

This commit moves away from caching on Travis to our own caching on S3 for
caching docker layers between builds. Unfortunately the Travis caches have over
time had a few critical pain points:

* Caches are only updated for successful builds, meaning that if a build times
  out or fails in a different location the sucessfully-created docker images
  isn't always cached. While this makes sense as a general rule of caches it
  hurts our use cases.

* Caches are per-branch and builder which means that we don't have a separate
  cache on each release channel. All our merges go through the `auto` branch
  which means that they're all sharing the same cache, even those for merging to
  master/beta. This means that PRs which switch between master/beta will keep
  rebuilting and having cache misses.

* Caches have historically been invaliated somewhat regularly a little more
  aggressively than we'd want (I think).

* We don't always need to update the contents of the cache if the Docker image
  didn't change at all, and saving off the docker layers can sometimes be quite
  expensive.

For all these reasons this commit drops the usage of Travis's built-in caching
support. Instead our own caching is used by storing blobs to S3. Normally this
would be a very risky endeavour but we're basically priming a cache for a cache
(docker) so if we get this wrong the failure mode is longer builds, not stale
caches. We'll notice that pretty quickly and hopefully fix it!

The logic here is inserted directly into the `src/ci/docker/run.sh` script to
download an image based on a shasum of the `Dockerfile` and other assorted files.
This blob, if found, is loaded into docker and we record what layers were
inserted. After docker finishes the build (hopefully quickly with lots of cache
hits) we then see the sha of the final image. If it's one of the layers we
loaded then there's no need to update the cache. Otherwise we upload our layers
to the global cache, possibly overwriting what we previously just downloaded.

This is hopefully a step towards mitigating #49278 although it doesn't
completely fix it as it means we'll still probably have to retry builds that
bust the cache.

.travis.yml

@@ -183,7 +183,6 @@ matrix:
       if: branch = master AND type = push
       before_install: []
       install: []
-      cache: false
       sudo: false
       script:
         MESSAGE_FILE=$(mktemp -t msg.XXXXXX);
@@ -201,7 +200,12 @@ env:
     - secure: "cFh8thThqEJLC98XKI5pfqflUzOlxsYPRW20AWRaYOOgYHPTiGWypTXiPbGSKaeAXTZoOA+DpQtEmefc0U6lt9dHc7a/MIaK6isFurjlnKYiLOeTruzyu1z7PWCeZ/jKXsU2RK/88DBtlNwfMdaMIeuKj14IVfpepPPL71ETbuk="
 
 before_install:
-  - zcat $HOME/docker/rust-ci.tar.gz | docker load || true
+  # We'll use the AWS cli to download/upload cached docker layers, so install
+  # that here.
+  - if [ "$TRAVIS_OS_NAME" = linux ]; then
+      pip install --user awscli;
+      export PATH=$PATH:$HOME/.local/bin;
+    fi
   - mkdir -p $HOME/rustsrc
   # FIXME(#46924): these two commands are required to enable IPv6,
   # they shouldn't exist, please revert once more official solutions appeared.
@@ -286,23 +290,9 @@ after_failure:
   # it happened
   - dmesg | grep -i kill
 
-# Save tagged docker images we created and load them if they're available
-# Travis saves caches whether the build failed or not, nuke rustsrc if
-# the failure was while updating it (as it may be in a bad state)
-# https://github.com/travis-ci/travis-ci/issues/4472
-before_cache:
-  - docker history -q rust-ci |
-    grep -v missing |
-    xargs docker save |
-    gzip > $HOME/docker/rust-ci.tar.gz
-
 notifications:
   email: false
 
-cache:
-  directories:
-    - $HOME/docker
-
 before_deploy:
   - mkdir -p deploy/$TRAVIS_COMMIT
   - >

src/ci/docker/run.sh

@@ -27,6 +27,21 @@ travis_fold start build_docker
 travis_time_start
 
 if [ -f "$docker_dir/$image/Dockerfile" ]; then
+    if [ "$CI" != "" ]; then
+      cksum=$(find $docker_dir/$image $docker_dir/scripts -type f | \
+        sort | \
+        xargs cat | \
+        sha512sum | \
+        awk '{print $1}')
+      s3url="s3://$SCCACHE_BUCKET/docker/$cksum"
+      url="https://s3-us-west-1.amazonaws.com/$SCCACHE_BUCKET/docker/$cksum"
+      echo "Attempting to download $s3url"
+      set +e
+      loaded_images=$(curl $url | docker load | sed 's/.* sha/sha/')
+      set -e
+      echo "Downloaded containers:\n$loaded_images"
+    fi
+
     dockerfile="$docker_dir/$image/Dockerfile"
     if [ -x /usr/bin/cygpath ]; then
         context="`cygpath -w $docker_dir`"
@@ -40,6 +55,23 @@ if [ -f "$docker_dir/$image/Dockerfile" ]; then
       -t rust-ci \
       -f "$dockerfile" \
       "$context"
+
+    if [ "$s3url" != "" ]; then
+      digest=$(docker inspect rust-ci --format '{{.Id}}')
+      echo "Built container $digest"
+      if ! grep -q "$digest" <(echo "$loaded_images"); then
+        echo "Uploading finished image to $s3url"
+        set +e
+        docker history -q rust-ci | \
+          grep -v missing | \
+          xargs docker save | \
+          gzip | \
+          aws s3 cp - $s3url
+        set -e
+      else
+        echo "Looks like docker image is the same as before, not uploading"
+      fi
+    fi
 elif [ -f "$docker_dir/disabled/$image/Dockerfile" ]; then
     if [ -n "$TRAVIS_OS_NAME" ]; then
         echo Cannot run disabled images on travis!