Fix some escaping around env var processing
Clarify in the docs that ENV is not recursive
Closes
@@ -146,6 +146,17 @@ The instructions that handle environment variables in the `Dockerfile` are:
`ONBUILD` instructions are **NOT** supported for environment replacement, even
the instructions above.
+Environment variable subtitution will use the same value for each variable
+throughout the entire command. In other words, in this example:
+
+ ENV abc=hello
+ ENV abc=bye def=$abc
+ ENV ghi=$abc
+
+will result in `def` having a value of `hello`, not `bye`. However,
+`ghi` will have a value of `bye` because it is not part of the same command
+that set `abc` to `bye`.
+
## The `.dockerignore` file
If a file named `.dockerignore` exists in the source repository, then it
