Use a deterministic target path for feature dir mounts with useBuildContexts

This avoids caching issues when using the Dockerfile produced in
useBuildContexts mode. See
#205 (comment) for
context.

Author: aaronlehmann

devcontainer/devcontainer_test.go

@@ -131,13 +131,13 @@ FROM localhost:5000/envbuilder-test-codercom-code-server:latest
 
 USER root
 # Rust tomato - Example description!
-WORKDIR `+featureOneDir+`
+WORKDIR /envbuilder_features/one
 ENV TOMATO=example
-RUN --mount=type=bind,from=envbuilder_feature_one,target=`+featureOneDir+`,rw _CONTAINER_USER="1000" _REMOTE_USER="1000" ./install.sh
+RUN --mount=type=bind,from=envbuilder_feature_one,target=/envbuilder_features/one,rw _CONTAINER_USER="1000" _REMOTE_USER="1000" ./install.sh
 # Go potato - Example description!
-WORKDIR `+featureTwoDir+`
+WORKDIR /envbuilder_features/two
 ENV POTATO=example
-RUN --mount=type=bind,from=envbuilder_feature_two,target=`+featureTwoDir+`,rw VERSION="potato" _CONTAINER_USER="1000" _REMOTE_USER="1000" ./install.sh
+RUN --mount=type=bind,from=envbuilder_feature_two,target=/envbuilder_features/two,rw VERSION="potato" _CONTAINER_USER="1000" _REMOTE_USER="1000" ./install.sh
 USER 1000`, params.DockerfileContent)
 
 		require.Equal(t, map[string]string{

devcontainer/features/features.go

@@ -218,6 +218,8 @@ func (s *Spec) Compile(featureRef, featureName, featureDir, containerUser, remot
 	sort.Strings(runDirective)
 	// See https://containers.dev/implementors/features/#invoking-installsh
 	if useBuildContexts {
+		// Use a deterministic target directory to make the resulting Dockerfile cacheable
+		featureDir = "/envbuilder_features/" + featureName
 		fromDirective = "FROM scratch AS envbuilder_feature_" + featureName + "\nCOPY --from=" + featureRef + " / /\n"
 		runDirective = append([]string{"RUN", "--mount=type=bind,from=envbuilder_feature_" + featureName + ",target=" + featureDir + ",rw"}, runDirective...)
 	} else {