Fix master builds (#331)

# Summary

Master patches are broken because they build with `latest` tag only,
while the patch still need the `patch_id` tagged images.
This PR fixes this issue to get master green again, but remove builds to
`latest`.
They will be added again in
#317, using
build_info.
Not pushing to latest only impacts local testing for dev, as we use them
to have the most recently built versions from master. It won't break
local development, but it means the image used will be slightly
outdated.

## Proof of Work
I tested it by forcing to return the patch ID and dumping the relevant
variables in this patch:

https://spruce.mongodb.com/version/6899d033f94ad4000694cf89/tasks?sorts=STATUS%3AASC%3BBASE_STATUS%3ADESC

But the relevant environment variable (`is_patch` being false, as
described in the added documentation) will be set to the right value
only when merging to master.

Author: Julien-Ben

scripts/release/build_context.py

@@ -17,21 +17,36 @@ class BuildScenario(str, Enum):
     @classmethod
     def infer_scenario_from_environment(cls) -> "BuildScenario":
         """Infer the build scenario from environment variables."""
+        # triggered_by_git_tag is the name of the tag that triggered this version, if applicable. It will be None for
+        # all patches except when tagging a commit on GitHub
         git_tag = os.getenv("triggered_by_git_tag")
+        # is_patch is passed automatically by Evergreen.
+        # It is "true" if the running task is in a patch build and undefined if it is not.
+        # A patch build is a version not triggered by a commit to a repository.
+        # It either runs tasks on a base commit plus some diff if submitted by the CLI or on a git branch if created by
+        # a GitHub pull request.
         is_patch = os.getenv("is_patch", "false").lower() == "true"
+        # RUNNING_IN_EVG is set by us in evg-private-context
         is_evg = os.getenv("RUNNING_IN_EVG", "false").lower() == "true"
+        # version_id is the id of the task's version. It is generated automatically for each task run.
+        # For example: `6899b7e35bfaee00077db986` for a manual/PR patch,
+        # or `mongodb_kubernetes_5c5a3accb47bb411682b8c67f225b61f7ad5a619` for a master merge
         patch_id = os.getenv("version_id")
 
+        logger.debug(
+            f"Collected environment variables: git tag {git_tag}, is_patch {is_patch}, is_evg {is_evg}, patch_id {patch_id}"
+        )
+
         if git_tag:
             # Release scenario and the git tag will be used for promotion process only
             scenario = BuildScenario.RELEASE
-            logger.info(f"Build scenario: {scenario} (git_tag: {git_tag})")
+            logger.info(f"Build scenario: {scenario}, git_tag: {git_tag}")
         elif is_patch:
             scenario = BuildScenario.PATCH
-            logger.info(f"Build scenario: {scenario} (patch_id: {patch_id})")
+            logger.info(f"Build scenario: {scenario}, patch_id: {patch_id}")
         elif is_evg:
             scenario = BuildScenario.STAGING
-            logger.info(f"Build scenario: {scenario} (patch_id: {patch_id})")
+            logger.info(f"Build scenario: {scenario}, patch_id: {patch_id}")
         else:
             scenario = BuildScenario.DEVELOPMENT
             logger.info(f"Build scenario: {scenario}")
@@ -70,11 +85,15 @@ def get_version(self) -> str:
         if self.scenario == BuildScenario.RELEASE:
             return self.git_tag
         if self.scenario == BuildScenario.STAGING:
-            # On master merges, always use "latest" (preserving legacy behavior)
-            return "latest"
+            # On master merges, we need to build images with the patch_id as they are expected by tests. Later on,
+            # we will use commit SHA. Optionally, we may want to continue pushing to ECR registry with "latest", for
+            # local dev purposes.
+            return self.patch_id
         if self.patch_id:
             return self.patch_id
         # Alternatively, we can fail here if no ID is explicitly defined
+        # When working locally, "version_id" env variable is defined in the generated context file. It is "latest" by
+        # default, and can be overridden with OVERRIDE_VERSION_ID
         return "latest"
 
     def get_base_registry(self) -> str: