diff --git a/.dockerignore b/.dockerignore
index 4c49bd7..f66882e 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -1 +1,2 @@
 .env
+**/.idea/**
diff --git a/.gitignore b/.gitignore
index 7269f5a..5829b8a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,7 @@
+.env
 .idea/
 cacert-cats/
+cacert-gosigner/
 cacert-mgr/
 cacert-software/
-.env
 testca/
diff --git a/.mrconfig b/.mrconfig
index 941a88e..b2f6f56 100644
--- a/.mrconfig
+++ b/.mrconfig
@@ -6,3 +6,6 @@ checkout = git clone git+ssh://git.cacert.org/srv/git/cacert-mgr.git cacert-mgr
 
 [cacert-software]
 checkout = git clone git+ssh://git.cacert.org/srv/git/cacert-devel.git cacert-software
+
+[cacert-gosigner]
+checkout = git clone https://git.dittberner.info/jan/cacert-gosigner.git
diff --git a/docker-compose.yml b/docker-compose.yml
index 67cab0e..dd5d909 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -108,7 +108,7 @@ services:
   signer:
     build:
       context: .
-      dockerfile: signer.Dockerfile
+      dockerfile: gosigner.Dockerfile
     environment:
       SIGNER_WORKDIR: /srv/ca/work
       SIGNER_CA_CONFIG: /srv/caconfig
diff --git a/docker/run-gosigner b/docker/run-gosigner
new file mode 100755
index 0000000..44f7a7d
--- /dev/null
+++ b/docker/run-gosigner
@@ -0,0 +1,31 @@
+#!/bin/sh
+
+set -eu
+
+export SERIAL_PORT=/dev/ttyUSB0
+
+mkdir -p /srv/ca/CA/certs /srv/ca/CA/private /srv/ca/CA/newcerts
+cp /srv/testca/root/ca.crt.pem /srv/ca/CA/ca.crt.pem
+cp /srv/testca/root/private/ca.key.pem /srv/ca/CA/private/ca.key.pem
+if [ ! -f /srv/ca/CA/index.txt ]; then cp /srv/testca/root/index.txt /srv/ca/CA/index.txt; fi
+if [ ! -f /srv/ca/CA/index.txt.attr ]; then cp /srv/testca/root/index.txt.attr /srv/ca/CA/index.txt.attr; fi
+if [ ! -f /srv/ca/CA/serial ]; then printf '00' >/srv/ca/CA/serial; fi
+if [ ! -f /srv/ca/CA/crlnumber ]; then echo 1000 >/srv/ca/CA/crlnumber; fi
+
+mkdir -p /srv/ca/class3/certs /srv/ca/class3/private /srv/ca/class3/newcerts /srv/ca/gpg/gpg_root_0
+cp /srv/testca/class3/ca.crt.pem /srv/ca/class3/ca.crt.pem
+cp /srv/testca/class3/private/ca.key.pem /srv/ca/class3/private/ca.key.pem
+if [ ! -f /srv/ca/class3/index.txt ]; then cp /srv/testca/class3/index.txt /srv/ca/class3/index.txt; fi
+if [ ! -f /srv/ca/class3/index.txt.attr ]; then cp /srv/testca/class3/index.txt.attr /srv/ca/class3/index.txt.attr; fi
+if [ ! -f /srv/ca/class3/serial ]; then printf '00' >/srv/ca/class3/serial; fi
+if [ ! -f /srv/ca/class3/crlnumber ]; then echo 1000 >/srv/ca/class3/crlnumber; fi
+if [ ! -f /srv/ca/gpg/gpg_root_0/secring.gpg ]; then cp /srv/testca/gpg/gpg_root_0/secring.gpg /srv/ca/gpg/gpg_root_0/secring.gpg; fi
+if [ ! -f /srv/ca/gpg/gpg_root_0/pubring.gpg ]; then cp /srv/testca/gpg/gpg_root_0/pubring.gpg /srv/ca/gpg/gpg_root_0/pubring.gpg; fi
+
+rm -f /srv/sockets/signer
+socat -d -d PTY,link=/dev/ttyUSB0 UNIX-LISTEN:/srv/sockets/signer 2>&1 &
+sleep 1
+
+cd /app/
+
+exec ./signer
diff --git a/docker/signer-config/class3-client-codesign.cnf b/docker/signer-config/class3-client-codesign.cnf
index c6713c3..4d29fab 100644
--- a/docker/signer-config/class3-client-codesign.cnf
+++ b/docker/signer-config/class3-client-codesign.cnf
@@ -29,10 +29,10 @@ commonName             = optional
 emailAddress           = optional
 
 [ usr_cert ]
-basicConstraints       = critical, CA:FALSE
+basicConstraints       = critical,CA:FALSE
 nsComment              = "To get your own certificate for FREE head over to http://www.CAcert.org"
-keyUsage               = critical, digitalSignature, keyEncipherment, keyAgreement
-extendedKeyUsage       = emailProtection, clientAuth, codeSigning, msCodeInd, msCodeCom, msEFS, msSGC, nsSGC
+keyUsage               = critical,digitalSignature,keyEncipherment,keyAgreement
+extendedKeyUsage       = emailProtection,clientAuth,codeSigning,msCodeInd,msCodeCom,msEFS,msSGC,nsSGC
 authorityInfoAccess    = OCSP;URI:http://ocsp.cacert.org
 crlDistributionPoints  = URI:http://crl.cacert.localhost/test-class3-revoke.crl
 subjectAltName         = email:copy
diff --git a/docker/signer-config/class3-client-org.cnf b/docker/signer-config/class3-client-org.cnf
index ebb09a2..eef5545 100644
--- a/docker/signer-config/class3-client-org.cnf
+++ b/docker/signer-config/class3-client-org.cnf
@@ -29,10 +29,10 @@ commonName             = optional
 emailAddress           = optional
 
 [ usr_cert ]
-basicConstraints       = critical, CA:FALSE
+basicConstraints       = critical,CA:FALSE
 nsComment              = "To get your own certificate for FREE head over to http://www.CAcert.org"
-keyUsage               = critical, digitalSignature, keyEncipherment, keyAgreement
-extendedKeyUsage       = emailProtection, clientAuth, msEFS, msSGC, nsSGC
+keyUsage               = critical,digitalSignature,keyEncipherment,keyAgreement
+extendedKeyUsage       = emailProtection,clientAuth,msEFS,msSGC,nsSGC
 authorityInfoAccess    = OCSP;URI:http://ocsp.cacert.org
 crlDistributionPoints  = URI:http://crl.cacert.localhost/class3-revoke.crl
 subjectAltName         = email:copy
diff --git a/docker/signer-config/class3-client.cnf b/docker/signer-config/class3-client.cnf
index b9624f8..b800583 100644
--- a/docker/signer-config/class3-client.cnf
+++ b/docker/signer-config/class3-client.cnf
@@ -30,10 +30,10 @@ commonName             = optional
 emailAddress           = optional
 
 [ usr_cert ]
-basicConstraints       = critical, CA:FALSE
+basicConstraints       = critical,CA:FALSE
 nsComment              = "To get your own certificate for FREE head over to http://www.CAcert.org"
-keyUsage               = critical, digitalSignature, keyEncipherment, keyAgreement
-extendedKeyUsage       = emailProtection, clientAuth, msEFS, msSGC, nsSGC
+keyUsage               = critical,digitalSignature,keyEncipherment,keyAgreement
+extendedKeyUsage       = emailProtection,clientAuth,msEFS,msSGC,nsSGC
 authorityInfoAccess    = OCSP;URI:http://ocsp.cacert.org
 crlDistributionPoints  = URI:http://crl.cacert.localhost/class3-revoke.crl
 subjectAltName         = email:copy
diff --git a/gosigner.Dockerfile b/gosigner.Dockerfile
new file mode 100644
index 0000000..09ed63e
--- /dev/null
+++ b/gosigner.Dockerfile
@@ -0,0 +1,30 @@
+FROM golang:alpine AS builder
+
+COPY cacert-gosigner /src/
+
+RUN set -ex ; \
+    cd /src/cmd/signer ; \
+    CGO_ENABLED=0 go build .
+
+# need to use Debian because we need xdelta 1.x to support signer protocol
+# version 1
+FROM debian:buster-slim
+
+RUN apt-get update \
+    && DEBIAN_FRONTEND=noninteractive \
+    apt-get install -y --no-install-recommends \
+    socat \
+    xdelta \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY --from=builder /src/cmd/signer/signer /app/
+
+COPY docker/run-gosigner usr/local/bin/
+COPY docker/signer-config/* /srv/caconfig/
+COPY testca /srv/testca/
+
+VOLUME /srv/ca
+
+ENTRYPOINT []
+CMD ["/usr/local/bin/run-gosigner"]
diff --git a/imap.Dockerfile b/imap.Dockerfile
deleted file mode 100644
index 14859a5..0000000
--- a/imap.Dockerfile
+++ /dev/null
@@ -1,15 +0,0 @@
-FROM debian:buster
-
-RUN apt-get update \
-    && DEBIAN_FRONTEND=noninteractive \
-    apt-get install -y --no-install-recommends \
-    ca-certificates \
-    curl \
-    dovecot-imapd \
-    psmisc \
-    && apt-get clean \
-    && rm -rf /var/lib/apt/lists/*
-
-EXPOSE 25
-
-COPY docker/start-postfix.sh /usr/local
\ No newline at end of file
diff --git a/mail.Dockerfile b/mail.Dockerfile
index 2ea32fb..9ff5fd9 100644
--- a/mail.Dockerfile
+++ b/mail.Dockerfile
@@ -1,4 +1,4 @@
-FROM debian:buster
+FROM debian:buster-slim
 
 RUN apt-get update \
     && DEBIAN_FRONTEND=noninteractive \
diff --git a/setup_test_ca.sh b/setup_test_ca.sh
index 9a9a3b3..4c7ea0a 100755
--- a/setup_test_ca.sh
+++ b/setup_test_ca.sh
@@ -133,8 +133,7 @@ if [ ! -f root/ca.crt.pem ]; then
   openssl req -new -x509 -config ca.cnf \
     -keyout root/private/ca.key.pem \
     -nodes \
-    -subj "/CN=Test Root/C=${COUNTRY_CODE}/O=${ORGANIZATION}" \
-    -days 3650 \
+    -subj "/C=${COUNTRY_CODE}/O=${ORGANIZATION}/CN=Test Root" \
     -extensions root_extensions \
     -out root/ca.crt.pem
 fi
diff --git a/smtp.Dockerfile b/smtp.Dockerfile
index 39a9195..8ed5da3 100644
--- a/smtp.Dockerfile
+++ b/smtp.Dockerfile
@@ -1,4 +1,4 @@
-FROM debian:buster
+FROM debian:buster-slim
 
 RUN apt-get update \
     && DEBIAN_FRONTEND=noninteractive \
@@ -26,4 +26,4 @@ VOLUME /home/catchall/Maildir
 
 COPY docker/run-postfix /usr/local/bin/run-postfix
 
-CMD ["dumb-init", "/usr/local/bin/run-postfix"]
\ No newline at end of file
+CMD ["dumb-init", "/usr/local/bin/run-postfix"]