Dockerfiles moved to root for shared libraries (flutter#4071)

Fixes locally what was previously failing

```
> docker build -t test -f Dockerfile.app_dart

Downloading packages...
...
+ cocoon_server 0.0.0 from path ../cocoon_server
```

app_dart/Dockerfile → Dockerfile.app_dart

@@ -6,10 +6,15 @@
 # Dart Docker official images can be found here: https://hub.docker.com/_/dart
 FROM dart:beta@sha256:8f9ccb89e68772720fbedf48baf0f6cc1bf61a6d1fd9962c3a418adbd89bdca0
 
+WORKDIR /cocoon_server
+
+COPY cocoon_server .
+
 WORKDIR /app
 
 # Copy app source code (except anything in .dockerignore).
-COPY . .
+COPY app_dart .
+
 RUN dart pub get
 
 # Start server.

auto_submit/Dockerfile → Dockerfile.auto_submit

@@ -6,10 +6,14 @@
 # Dart Docker official images can be found here: https://hub.docker.com/_/dart
 FROM dart:beta@sha256:8f9ccb89e68772720fbedf48baf0f6cc1bf61a6d1fd9962c3a418adbd89bdca0
 
+WORKDIR /cocoon_server
+
+COPY cocoon_server .
+
 WORKDIR /app
 
 # Copy app source code (except anything in .dockerignore).
-COPY . .
+COPY auto_submit .
 RUN dart pub get
 
 # Start server.

app_dart/cloudbuild_app_dart.yaml

@@ -18,7 +18,7 @@ steps:
 
   # Build docker image
   - name: 'us-docker.pkg.dev/cloud-builders/ga/v1/docker'
-    args: ['build', '-t', 'us-docker.pkg.dev/$PROJECT_ID/appengine/default.version-$SHORT_SHA', 'app_dart']
+    args: ['build', '-t', 'us-docker.pkg.dev/$PROJECT_ID/appengine/default.version-$SHORT_SHA', '-f', 'Dockerfile.app_dart']
 
   # Trigger the cloud build that deploys the docker image
   - name: gcr.io/cloud-builders/gcloud

auto_submit/cloudbuild_auto_submit.yaml

@@ -10,7 +10,7 @@
 steps:
   # Build docker image
   - name: 'us-docker.pkg.dev/cloud-builders/ga/v1/docker'
-    args: ['build', '-t', 'us-docker.pkg.dev/$PROJECT_ID/appengine/auto-submit.version-$SHORT_SHA', 'auto_submit']
+    args: ['build', '-t', 'us-docker.pkg.dev/$PROJECT_ID/appengine/auto-submit.version-$SHORT_SHA', '-f', 'Dockerfile.auto_submit']
 
   # Trigger the cloud build that deploys the docker image
   - name: gcr.io/cloud-builders/gcloud