AWS App Runner start command with quoted whitespaces

Question

I'm creating an AppRunner service from a private ECR image with a Start command:

--before 'echo $MYVAR > /tmp/log.txt'

Which should be passed as is into my container entrypoint.

However, in App Runner application logs I get an error:

Couldn't find match for argument PARSE ERROR: Argument: $MYVAR

If I run the image using just Docker the whole 'echo $MYVAR > /tmp/log.txt' is interpreted as one argument to --before.

But in the App Runner two arguments are passed to the entrypoint:

• --before with 'echo value
• $MYVAR > /tmp/log.txt'

Second argument cases a parsing error in the entrypoint since it's invalid.

I have tried various ways of specifying an entrypoint but they all fail:

--before "echo $MYVAR > /tmp/log.txt"
--before \'echo $MYVAR > /tmp/log.txt\'

Is there any way I can pass 'echo $MYVAR > /tmp/log.txt' ass one argument to --before in App Runner start command?