Merge pull request moby#5126 from dvdksn/docs-improve-LegacyKeyValueFormat

docs: add a multi-line ENV example

Author: tonistiigi

frontend/dockerfile/docs/rules/legacy-key-value-format.md

@@ -36,3 +36,21 @@ FROM alpine
 ARG foo=bar
 ```
 
+❌ Bad: multi-line variable declaration with a space separator.
+
+```dockerfile
+ENV DEPS \
+    curl \
+    git \
+    make
+```
+
+✅ Good: use an equals sign and wrap the value in quotes.
+
+```dockerfile
+ENV DEPS="\
+    curl \
+    git \
+    make"
+```
+

frontend/dockerfile/linter/docs/LegacyKeyValueFormat.md

@@ -28,3 +28,21 @@ ARG foo bar
 FROM alpine
 ARG foo=bar
 ```
+
+❌ Bad: multi-line variable declaration with a space separator.
+
+```dockerfile
+ENV DEPS \
+    curl \
+    git \
+    make
+```
+
+✅ Good: use an equals sign and wrap the value in quotes.
+
+```dockerfile
+ENV DEPS="\
+    curl \
+    git \
+    make"
+```