There are a lot of examples here how to source a dotenv file in bash but has anyone one a method that achieves the same with dash (which is the default shell for minimal Debian installations)?
The solution should look like this:
$ some foo my-command-using-env-vars
e.g.
$ env $(cat .env) my-command-using-env-vars
And it is important that the solution supports multiline values with spaces like:
SSH_PRIVATE_KEY="-----BEGIN RSA PRIVATE KEY-----\nfoo\nbar\baz"
and special characters like hash within quotes:
SPECIAL="foo#bar"
It seems that your problem is not so much that you're using dash but that you want to support \n escapes. The following works in dash and supports \n escapes:
eval "$(echo $(cat .env))" my-command-using-env-vars
That's because unlike in bash the built-in echo in dash supports backslash escapes even without the -e option. The following works in both bash and dash, provided that the non-built-in, system echo supports the -e option:
eval "$(env echo -e $(cat .env))" my-command-using-env-vars
Note that both approaches will also handle other backslash escapes (either POSIX or GNU), possibly in a different way than you expect.
Some technical notes:
$(cat .env)
performs Field Splitting, converting any newline in file .env into spaces.
"$(env echo -e ...)"
expands backslash escapes regardless of the current shell by invoking echo -e via env. The double quotes disable field splitting, so that newlines are preserved.