Use the linux-micro image for tests

I tested that this can build `hexponent 0.3.0` successfully. This cuts
the download size down from over 1 GB to about 150 MB.

Author: jyn514

.env.sample

@@ -5,3 +5,4 @@ AWS_ACCESS_KEY_ID=cratesfyi
 AWS_SECRET_ACCESS_KEY=secret_key
 S3_ENDPOINT=http://localhost:9000
 DOCSRS_INCLUDE_DEFAULT_TARGETS=false
+DOCSRS_DOCKER_IMAGE=ghcr.io/rust-lang/crates-build-env/linux-micro

.github/workflows/ci.yml

@@ -38,14 +38,10 @@ jobs:
       - name: fast tests
         run: cargo test --workspace --locked
 
-      - name: create small build-environment
-        run: |
-          docker build -t buildenv - < dockerfiles/Dockerfile-small-build-env
-
       - name: slow tests
         env:
           DOCSRS_INCLUDE_DEFAULT_TARGETS: true
-          DOCS_RS_LOCAL_DOCKER_IMAGE: buildenv
+          DOCSRS_DOCKER_IMAGE: ghcr.io/rust-lang/crates-build-env/linux-micro
         run: cargo test --workspace --locked -- --ignored
 
       - name: Clean up the database