docker: Create second file for running tests

Dockerfile is now Dockerfile.cache which will be run approximately
weekly and will contain the cache of the current tools. New file
Dockerfile.run will start from that image and copy in the current
workspace.

Change-Id: If9c0311f7122fbed1f5e86e4d6391c0a1382765f
Bug: 216
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/15040
Reviewed-by: Johan Euphrosine <proppy@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Rob Mohr <mohrr@google.com>
diff --git a/docker/Dockerfile.run b/docker/Dockerfile.run
new file mode 100644
index 0000000..3cd21b3
--- /dev/null
+++ b/docker/Dockerfile.run
@@ -0,0 +1,21 @@
+# Copyright 2020 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+ARG from
+FROM ${from}
+
+WORKDIR /workspace
+COPY . /workspace
+
+CMD /bin/bash