Build snaps using custom Docker image

* Added --snapcraft option to release-tool build command

ci/snapcraft/Dockerfile

@@ -0,0 +1,51 @@
+# KeePassXC Linux Release Build Dockerfile
+# Copyright (C) 2017-2018 KeePassXC team <https://keepassxc.org/>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 2 or (at your option)
+# version 3 of the License.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+FROM snapcore/snapcraft
+
+ENV REBUILD_COUNTER=1
+
+ENV QT5_VERSION=510
+ENV QT5_PPA_VERSION=5.10.1
+
+RUN set -x \
+    && apt update -y \
+    && apt -y install software-properties-common
+
+RUN set -x \
+    && add-apt-repository ppa:phoerious/keepassxc
+
+RUN set -x \
+    && apt update -y \
+    && apt-get -y --no-install-recommends install \
+      build-essential \
+      cmake \
+      libgcrypt20-18-dev \
+      libargon2-0-dev \
+      libsodium-dev \
+      qtbase5-dev \
+      qttools5-dev \
+      qttools5-dev-tools \
+      zlib1g-dev \
+      libyubikey-dev \
+      libykpers-1-dev \
+      libxi-dev \
+      libxtst-dev \
+      xvfb
+
+RUN set -x \
+    && apt-get autoremove --purge
+