Generate them on your own instead of relying on Phoenix. Makefile for that:

NAME     = localhost
DOMAIN   = *.localhost

KEY      = ${NAME}.key
SIGN_REQ = ${NAME}.csr
CERT     = ${NAME}.crt

SUBJECT = "/C=US/ST=Connecticut/O=/localityName=New Haven/commonName=$(DOMAIN)/commonName=localhost/organizationalUnitName=/emailAddress=/"

all: ${CERT}

clean:
	$(RM) -rf ${KEY} ${CERT} ${SIGN_REQ}

verify: ${CERT}
	openssl x509 -noout -text -in $<

${KEY}:
	openssl genrsa -out "$@" 2048

${SIGN_REQ}: ${KEY}
	openssl req -new -sha256 -subj $(SUBJECT) -key "$<" -out "$@" -passin pass:""

${CERT}: ${SIGN_REQ} ${KEY}
	openssl x509 -req -days 365 -in "${SIGN_REQ}" -signkey "${KEY}" -out "$@"

.PHONY: all clean verify