ENV depending on each other in a single layer

[caarlos0]

If I set multiple variables in one layer, with one variable depending on the value of the other, it would not work.

Example Dockerfile:

FROM alpine
ENV VAR1="VALUE" \
  VAR2="VAL 1 = $VAR1"
ENTRYPOINT env | grep VAR

Running:

$ docker build -t vars .
# ...
$ docker run -t vars
VAR1=VAR 1 VALUE
VAR2=VAL 1 = 

Tested on Docker 1.6.2 and 1.7.1.

---

docker version:

Client version: 1.7.1
Client API version: 1.19
Go version (client): go1.4.2
Git commit (client): 786b29d
OS/Arch (client): linux/amd64
Server version: 1.7.1
Server API version: 1.19
Go version (server): go1.4.2
Git commit (server): 786b29d
OS/Arch (server): linux/amd64

docker info:

Containers: 34
Images: 398
Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 466
 Dirperm1 Supported: true
Execution Driver: native-0.2
Logging Driver: json-file
Kernel Version: 3.19.0-21-generic
Operating System: Ubuntu 15.04
CPUs: 4
Total Memory: 7.704 GiB
Name: becker
ID: OBGW:LNVQ:XU5O:IXO3:G4RG:FIIN:TVGE:SZFL:L5IE:YFLQ:JILX:FAOQ
Username: caninjas
Registry: https://index.docker.io/v1/
WARNING: No swap limit support

uname -a

Linux becker 3.19.0-21-generic #21-Ubuntu SMP Sun Jun 14 18:31:11 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

[duglin]

at one point we almost let in a PR that could do that but then it was rejected because that's not a normal sh-ism. Would have been cool though :-) So, gonna close this as a dup - see: #10431 (comment)

ping @tianon if he has any extra comments