Always attempt to install prefect in base image

CHANGELOG.md

@@ -25,6 +25,7 @@ These changes are available in the [master branch](https://github.com/PrefectHQ/
 ### Fixes
 
 - Fix Fargate Agent access defaults and environment variable support - [#1687](https://github.com/PrefectHQ/prefect/pull/1687)
+- Attempt to install prefect in any docker image (if it is not already installed) - [#1704](https://github.com/PrefectHQ/prefect/pull/1704)
 
 ### Deprecations
 

src/prefect/environments/storage/docker.py

@@ -102,17 +102,20 @@ def __init__(
             else:
                 # create an image from python:*-slim directly
                 self.base_image = "python:{}-slim".format(python_version)
-                self.extra_commands.extend(
-                    [
-                        "apt update && apt install -y gcc git && rm -rf /var/lib/apt/lists/*",
-                        "pip install git+https://github.com/PrefectHQ/prefect.git@{}#egg=prefect[kubernetes]".format(
-                            self.prefect_version
-                        ),
-                    ]
+                self.extra_commands.append(
+                    "apt update && apt install -y gcc git && rm -rf /var/lib/apt/lists/*",
                 )
         else:
             self.base_image = base_image
 
+        # we should always try to install prefect, unless it is already installed. We can't determine this until
+        # image build time.
+        self.extra_commands.append(
+            "pip show prefect || pip install git+https://github.com/PrefectHQ/prefect.git@{}#egg=prefect[kubernetes]".format(
+                self.prefect_version
+            ),
+        )
+
         not_absolute = [
             file_path for file_path in self.files if not os.path.isabs(file_path)
         ]

tests/environments/storage/test_docker_storage.py

@@ -316,15 +316,15 @@ def test_create_dockerfile_from_base_image():
             "master",
             (
                 "FROM python:3.6-slim",
-                "pip install git+https://github.com/PrefectHQ/prefect.git@master",
+                "pip show prefect || pip install git+https://github.com/PrefectHQ/prefect.git@master",
             ),
         ),
         (
             "424be6b5ed8d3be85064de4b95b5c3d7cb665510",
             (
                 "FROM python:3.6-slim",
                 "apt update && apt install -y gcc git && rm -rf /var/lib/apt/lists/*",
-                "pip install git+https://github.com/PrefectHQ/prefect.git@424be6b5ed8d3be85064de4b95b5c3d7cb665510#egg=prefect[kubernetes]",
+                "pip show prefect || pip install git+https://github.com/PrefectHQ/prefect.git@424be6b5ed8d3be85064de4b95b5c3d7cb665510#egg=prefect[kubernetes]",
             ),
         ),
     ],