Change to recommended way of defining ENV

We are getting warnings that we are using the legacy way of defining
environement variables, so let's just switch to the new way. It
appears to work just fine like this as well.

Author: JonasAlfredsson

src/Dockerfile

@@ -1,7 +1,7 @@
 FROM nginx:1.27.0
 LABEL maintainer="Jonas Alfredsson <[email protected]>"
 
-ENV CERTBOT_DNS_AUTHENTICATORS \
+ENV CERTBOT_DNS_AUTHENTICATORS="\
     cloudflare \
     digitalocean \
     dnsimple \
@@ -17,7 +17,8 @@ ENV CERTBOT_DNS_AUTHENTICATORS \
     sakuracloud \
     ionos \
     bunny \
-    duckdns
+    duckdns \
+    "
 
 # Needed in order to install Python packages via PIP after PEP 668 was
 # introduced, but I believe this is safe since we are in a container without

src/Dockerfile-alpine

@@ -1,7 +1,7 @@
 FROM nginx:1.27.0-alpine
 LABEL maintainer="Jonas Alfredsson <[email protected]>"
 
-ENV CERTBOT_DNS_AUTHENTICATORS \
+ENV CERTBOT_DNS_AUTHENTICATORS="\
     cloudflare \
     digitalocean \
     dnsimple \
@@ -17,7 +17,8 @@ ENV CERTBOT_DNS_AUTHENTICATORS \
     sakuracloud \
     ionos \
     bunny \
-    duckdns
+    duckdns \
+    "
 
 # Needed in order to install Python packages via PIP after PEP 668 was
 # introduced, but I believe this is safe since we are in a container without