buildctl: propagate SOURCE_DATE_EPOCH from client env to build arg

Fix issue 4227

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cmd/buildctl/build/opt.go

@@ -1,5 +1,13 @@
 package build
 
 func ParseOpt(opts []string) (map[string]string, error) {
-	return attrMap(opts)
+	m := loadOptEnv()
+	m2, err := attrMap(opts)
+	if err != nil {
+		return nil, err
+	}
+	for k, v := range m2 {
+		m[k] = v
+	}
+	return m, nil
 }

cmd/buildctl/build/util.go

@@ -6,6 +6,7 @@ import (
 	"github.com/pkg/errors"
 
 	"github.com/moby/buildkit/client"
+	"github.com/moby/buildkit/util/bklog"
 )
 
 // loadGithubEnv verify that url and token attributes exists in the
@@ -31,3 +32,17 @@ func loadGithubEnv(cache client.CacheOptionsEntry) (client.CacheOptionsEntry, er
 	}
 	return cache, nil
 }
+
+// loadOptEnv loads opt values from the environment.
+// The returned map is always non-nil.
+func loadOptEnv() map[string]string {
+	m := make(map[string]string)
+	propagatableEnvs := []string{"SOURCE_DATE_EPOCH"}
+	for _, env := range propagatableEnvs {
+		if v, ok := os.LookupEnv(env); ok {
+			bklog.L.Debugf("Propagating %s from the client env to the build arg", env)
+			m["build-arg:"+env] = v
+		}
+	}
+	return m
+}

docs/build-repro.md

@@ -47,8 +47,10 @@ Minimal support is also available on older BuildKit when using Dockerfile 1.5 fr
 buildctl build --frontend dockerfile.v0 --opt build-arg:SOURCE_DATE_EPOCH=$(git log -1 --pretty=%ct) ...
 ```
 
-The `buildctl` CLI does not automatically propagate the `$SOURCE_DATE_EPOCH` environment value from the client host to the `SOURCE_DATE_EPOCH` build arg.
-However, higher level build tools, such as Docker Buildx (>= 0.10), may automatically capture the environment value.
+The `buildctl` CLI (<= 0.12) does not automatically propagate the `$SOURCE_DATE_EPOCH` environment value from the client host to the `SOURCE_DATE_EPOCH` build arg.
+<!-- TODO: s/master/v0.13/ -->
+In the `master` branch of BuildKit, the `buildctl` CLI is updated to automatically capture the environment value.
+Docker Buildx (>= 0.10) automatically captures the environment value too.
 
 The build arg value is used for:
 - the `created` timestamp in the [OCI Image Config](https://github.com/opencontainers/image-spec/blob/main/config.md#properties)