go-yaml load error in scanner (while scanning an alias) ... did not find expected alphabetic or numeric character means a value in your Compose file starts with an unquoted *. YAML reads * as the start of an alias name and finds no name after it. Put the value in quotes: "*".

The same message with (while scanning an anchor) comes from an unquoted &. Both characters have a meaning in YAML at the start of a value, and quoting turns them back into plain text.

The error

PS> docker compose -f compose.yaml config
go-yaml load error in scanner (while scanning an alias) at L7.C22-C23: did not find expected alphabetic or numeric character

The file, with a wildcard value for an environment variable:

name: fix10-cy-p8
services:
  web:
    image: alpine
    command: ["sleep", "1"]
    environment:
      ALLOWED_HOSTS: *

Why it happens

YAML lets you name a block with an anchor, &name, and reuse it elsewhere with an alias, *name. So when a value begins with *, the scanner reads an alias and expects a name made of letters or digits right after it. Here the line ends at column 23, so there is no name, and the scanner stops before Compose reads any service.

An & at the start of a value fails the same way, as an anchor without a name: 

PS> docker compose -f compose.amp.yaml config
go-yaml load error in scanner (while scanning an anchor) at L7.C18-C19: did not find expected alphabetic or numeric character

The fix

name: fix10-cy-p8
services:
  web:
    image: alpine
    command: ["sleep", "1"]
    environment:
      ALLOWED_HOSTS: "*"
PS> docker compose -f compose.fixed.yaml config
...
    environment:
      ALLOWED_HOSTS: '*'
...

In quotes the value is a one-character string, and config prints it back as '*', still quoted, because its output is YAML too. The quotes belong to the file, not to the value. A test service with one variable in double quotes and one in single quotes echoed both inside the container: 

    command: ["sh", "-c", "echo \"[$$ALLOWED_HOSTS] [$$SINGLE]\""]
    environment:
      ALLOWED_HOSTS: "*"
      SINGLE: '*'
PS> docker compose -f compose.run.yaml up
...
web-1  | [*] [*]
...

Both arrived as a plain *, so either kind of quote does the job.

How it was reproduced

Docker Desktop on Windows 11, Docker Compose v5.7.0, engine 29.9.3, Windows PowerShell 5.1. A one-service compose.yaml with ALLOWED_HOSTS: * under environment gave the message above on docker compose -f compose.yaml config with exit code 1, and a second file with SEPARATOR: & gave the anchor variant; with "*" the same command printed the model with exit code 0, and docker compose up on the test service with the cached alpine image printed the two values. The project name fix10-cy-p8 is the test's own.

Frequently asked

What does did not find expected alphabetic or numeric character mean in docker compose?
A value starts with an unquoted * or &. YAML reads them as an alias or an anchor and expects a name after them. Quote the value.
How do I set an environment variable to * in docker compose?
Write it in quotes, ALLOWED_HOSTS: "*". In the test config printed it back as '*' and the file was accepted.
What is the difference between scanning an alias and scanning an anchor?
The alias variant came from a value starting with *, the anchor variant from a value starting with &. The fix is the same: quote the value.

More decoded errors in the Fixes category. An alias that does have a name but points to no anchor gives unknown anchor referenced.