Nested variable substitution is unsupported

[schuelermine]

Describe the bug
Podman compose does not correctly handle variable substitution with default values or error messages that themselves contain variable substitutions.

To Reproduce
Steps to reproduce the behavior:

1. Use the following sample files:
   • Dockerfile:

     FROM registry.hub.docker.com/library/busybox:1.37.0-glibc
     
     ARG LINE1
     ARG LINE2
     ARG LINE3
     
     RUN <<EOF
     echo ${LINE1}
     echo ${LINE2}
     echo ${LINE3}
     EOF

   • compose.yaml:

     services:
       service:
         build:
           dockerfile: Dockerfile
           args:
             LINE1: "value of $$variable_1: $variable_1"
             LINE2: "value of $$variable_2: $variable_2"
             LINE3: "value of $${variable_1:-$${variable_2}}: ${variable_1:-${variable_2}}"

2. Run the following commands:
   • variable_1=foo podman compose build
   • variable_2=bar podman compose build

Expected behavior
The output of the third echo should end in:

• for variable_1=foo: foo
• for variable_2=bar: bar

Actual behavior
The relevant lines end with:

• for variable_1=foo: foo}
• for variable_2=bar: ${variable_2}

Output

$ podman compose version
>>>> Executing external compose provider "/run/current-system/sw/bin/podman-compose". Please see podman-compose(1) for how to disable this message. <<<<

podman-compose version 1.2.0
podman version 5.2.3
$ variable_1=foo podman compose build --no-cache
>>>> Executing external compose provider "/run/current-system/sw/bin/podman-compose". Please see podman-compose(1) for how to disable this message. <<<<

STEP 1/5: FROM registry.hub.docker.com/library/busybox:1.37.0-glibc
STEP 2/5: ARG LINE1
--> 330d7d829840
STEP 3/5: ARG LINE2
--> 4821dfb5862e
STEP 4/5: ARG LINE3
--> b5022f8e7e7f
STEP 5/5: RUN <<EOF (echo ${LINE1}...)
value of $variable_1: foo
value of $variable_2:
value of ${variable_1:-${variable_2}}: foo}
COMMIT goal-tree-rl_service
--> e305b923d654
Successfully tagged localhost/goal-tree-rl_service:latest
e305b923d654d30bacca9c10fb1d01501a799fbe86ceaf6571d0cd187b40e737
$ variable_2=bar podman compose build --no-cache
>>>> Executing external compose provider "/run/current-system/sw/bin/podman-compose". Please see podman-compose(1) for how to disable this message. <<<<

STEP 1/5: FROM registry.hub.docker.com/library/busybox:1.37.0-glibc
STEP 2/5: ARG LINE1
--> c8fbf7102206
STEP 3/5: ARG LINE2
--> c06fd0ab1eec
STEP 4/5: ARG LINE3
--> 3e9b51fe4501
STEP 5/5: RUN <<EOF (echo ${LINE1}...)
value of $variable_1:
value of $variable_2: bar
value of ${variable_1:-${variable_2}}: ${variable_2}
COMMIT goal-tree-rl_service
--> 77bdcc77f708
Successfully tagged localhost/goal-tree-rl_service:latest
77bdcc77f7081eeb79ebb78fbd5e7d6cbb01a2ee13718ef95efd1ff7f62ed684

Environment:

• OS: NixOS 24.11.20241023.2768c7d (Vicuna), Linux 6.11.5
• podman version: podman version 5.2.3
• podman compose version: git ref unknown; displays as 1.2.0

Additional context

Relevant section of Compose file specification: https://github.com/compose-spec/compose-spec/blob/main/12-interpolation.md